and the maternal ids are equivalent as regards the characters of the
species. This is the case, for instance, with the hybrids between the
two species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_,
which were reared by Kölreuter as far back as the eighteenth century,
and which then, as now, maintained a fairly exact medium between the
two ancestral species, and did so in all the individuals. Both species
thus strive to stamp their own character on the young plant, and in
both the hereditary power is equally great; in both it is contained
in the same number of ids, that is, in the half, for both kinds
of sex-cell have undergone reducing division. We have here, then,
strict proof that the half number of ids suffices to reproduce in the
offspring the type of the species, or, more generally, of the parents.

If we apply these results to the inheritance of individual differences
in Man, we may say, that those germ-cells, to which at the reducing
division the same combination of ids has been handed on as that which
already determined the type of the parent, will endeavour to impress
this image again on the child. If a female cell of this kind combine
with a male which likewise contains the facies-combination of the
parent, in this case the father, the same thing will happen which we
described in the case of the plant-hybrids, that is, a medium form
between the type of the two parents will arise.

Not infrequently, however, there is a marked preponderance of the one
parent in the type of the child, and we have to inquire whether the
theory gives us any help with regard to such a case.

One might be inclined to assume a difference in the determining power
of the paternal and maternal ids, but if we cannot show to what extent
and for what reason this power may be different such an assumption
remains rather an evasion than an explanation. Moreover, it would not
always apply to the conditions in Man, for if, for instance, the ids
of a particular mother were in general stronger than those of the
father, all the children of the pair in question would necessarily
take after the mother; but it happens not infrequently that one child
resembles the father preponderantly, and another the mother. Moreover,
the ids pass continually from the male to the female individual, and
conversely, by virtue of the continuity of the germ-plasm, so that the
idea that sex can have anything to do with the relative strength of the
ids is altogether erroneous.

But, as I have already said, unilateral inheritance occurs even in
the mingling of species-characters, and most clearly in the case of
plant-hybrids. Thus, for instance, hybrids between the two species of
pink, _Dianthus barbatus_ and _Dianthus deltoides_, resemble the latter
species much more closely than the former, and the hybrid between the
two species of foxglove which are wild in Germany, _Digitalis purpurea_
and _Digitalis lutea_, is much more like the latter than like the
former.

It might be reasonably asked whether, in these crossings, the normal
number of ids in one species is not greater than in the other. We
know that, among animals at least, differences in the normal number
of chromosomes occur even in very nearly related species. It is not
impossible that this, in many cases, is really the cause of the
diversity of transmitting power in different species. Nevertheless, we
cannot rest satisfied with this, for, in the first place, this cause
could not apply to the apparent unilateral inheritance from one parent
in Man, since the normal number of ids, as far as we know, is strictly
maintained in the same species, and second, this would not explain
certain phenomena of inheritance in plant-hybrids.

It happens not only frequently, but usually, that the different parts
of the hybrid take after one or other parent in different degree, and
this is the case also with children. In the hybrid between the two
species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_,
which I have already given as an example of a medium form between
the two parents, such diversities occur regularly in all the hybrid
individuals. Thus the corolla-tube of the hybrid is nearer _N.
paniculata_ in regard to its length, but nearer _N. rustica_ in regard
to its breadth. Many hybrids suggest one parent-form in the leaves, the
other in the blossoms. In the same way in the child the form of eye
may be that of the father, the colour of the iris that of the mother,
the nose maternal, the mouth paternal--in short, the preponderance in
heredity swings hither and thither from part to part, from organ to
organ, from character to character, and this is even the rule though
the oscillations may not always be apparent and are often invisible.

If we think of the proposition we arrived at earlier, and which was
proved chiefly by the case of identical twins, that the facies or
'type' of the descendant is determined at fertilization, we may be
inclined to regard such an oscillation of the hereditary tendencies
as almost impossible, for it means that, with the given mingling of
parental germ-plasms, the potency of inheritance from the two parents
in every part of the offspring is determined once for all in advance.
But the case of identical twins corroborates these oscillations, for
in them, too, the father predominates in one part, the mother in
another, and it proves, at the same time, that these oscillations do
not depend on any chances whatever in development, but that they are
exactly predetermined in the mingling of the hereditary substances in
the germ-plasm of the fertilized ovum, and are strictly adhered to
throughout development.

This fact can only be explained thus: the primary constituents of
the different parts and characters of the body are contained in the
parental germ-plasm in varying degrees of hereditary or transmissive
strength, and this can be understood very well from our point of view
without putting anything new into the 'portmanteau' of our theory
(Delage).

But I must digress a little in order to make this plain.

When, in speaking of plant-hybrids, I said that the collective ids
of the germ-plasm of a species must be equivalent in regard to the
characters of the species, I did not speak quite precisely; in the
majority of ids, in many cases in an overwhelming majority, this must
be the case, but not actually in all, at least not on the assumption
we make that the transformation of species is accomplished under the
control of natural selection.

Let us recall what we have already established in regard to the
evolutionary power of natural selection, namely, that the changes which
it controls can never transcend the range of their utility, and it will
be clear to us that, of the many ids which make up the germ-plasm of
the species, only so many will be modified as are necessary to evoke
the character which has varied. Just as the protective resemblance
of an insect to a leaf may be raised to a very high pitch, but can
never become perfect, because an imperfect resemblance is already
sufficient to deceive the persecutor, and the selective process comes
to a standstill because individuals which possessed a still greater
resemblance to a leaf would be no better protected from destruction
than the others, so in the modification of a species the whole of the
ids need not at once be modified, if a majority is sufficient to stamp
the great majority of individuals with the desired variation. But it
may happen that, at the reduction of ids during the development of the
germ-cells, an id-combination with wholly or almost wholly unchanged
ids may come together in one germ-cell, and if another sperm-cell of
this kind meets with an egg-cell similarly constituted, an individual
of the old species must arise. But this must--on our assumption--be
at a disadvantage as compared with the transformed individuals in the
struggle for existence, and will perish in it, and therefore the number
of unmodified ids in the germ-plasm of the species will gradually
diminish. It is obvious, however, that this will take place very
slowly, as we may conclude from the phenomena of reversion, of which I
shall have to speak later on.

But what is true of the ids is true also of their constituent parts,
the determinants, and that--if I mistake not--is fundamental in the
interpretation of the alternation of hereditary succession in the parts
of the child.

According to our theory, the ids do not collectively exert a
controlling influence on the cells, not even on the germ-cells, whose
histological differentiation into spermatic or egg-cells can only
depend on control through specific sex-cell determinants. It is the
different determinants of the ids that control; transformations of
the species will, it is true, depend on transformations of the ids,
but this need not necessarily consist in a variation of _all_ the
determinants of the id. If, for instance, two species of butterfly,
_Lycæna agestis_ in Germany and _Lycæna artaxerxes_ in Scotland, only
differ from each other in that the black spot in the middle of the wing
in _L. agestis_ is milk-white in _L. artaxerxes_, no other determinants
in the id of the germ-plasm can be different except those which control
this particular spot. In a majority of the ids in _L. artaxerxes_ the
determinants of this spot must have been modified, let us say, to the
production of 'milk-white.' This majority will increase very slowly if
the white colour has no pronounced advantage for the persistence of
the species, but it will increase gradually, as we have already seen,
though extremely slowly, through the elimination of those individuals
whose germ-plasm at the reducing division has chanced to receive a
majority of ids with the old, unmodified determinants, and which have
therefore reverted to the ancestral form. This will happen whenever the
new character has any use, however small, in maintaining the species.

But in most modifications of species quite a number of parts and
characters have undergone variation either simultaneously or in
rapid succession; in many cases nearly all the details of structure,
and therefore almost all the determinants of the germ-plasm, must
have varied. We must not, however, assume that all the equivalent
determinants, for instance, all the determinants _K_ in all the ids,
have varied[9], and above all we must not take for granted that
the determinants of different characters or parts of the body, for
instance, the determinants _L_, _M_, or _N_, must all undergo variation
in an equal number of ids. It will depend on two factors whether a new
character is implicit, in the form of varied determinants, in a small
or in a very large majority of ids: first, on the relative age of the
character, and, secondly, on its value in relation to the persistence
of the species. The more important a character is for the species the
more frequently is it decisive for the life or death of the individual,
and the more sharply will individuals not possessing it be eliminated,
and the more rapidly, therefore, will those whose germ-plasm still
contains a majority of the unvaried determinants of this character
tend to disappear. In this way these determinants will tend to sink
down from generation to generation to an ever smaller minority in the
germ-plasm of those that survive.

[9] By equivalent or homologous determinants I mean the determinants of
different ids which determine _similar parts_, e.g. the scales of that
wing-spot in _Lycæna agestis_ which is alluded to above, and to which
we must refer again in more detail.

Thus in the ids of any species which has been in some way
transformed--and that is as much as to say, in every species--the
equivalent or homologous determinants are modified in a very varied
percentage. A very modern and at the same time not very important
character _K´_ will only be contained in a small majority of ids,
while in the remainder the original homologous ancestral determinant
_K_ is contained; an older but not very much more important character
_M´_ must have its determinants in a larger majority in the ids, while
a character _V´_ of decisive importance for the preservation of the
species, if it has been in existence long enough, must be represented
in almost all the ids, so that the homologous unvaried determinants of
the ancestral species _V_ can only have persisted in an id here and
there.

If this argument be correct, many phenomena of inheritance become
intelligible, especially the variability in the expression of the
inheritance in the parts of the offspring, which is more or less
rigidly predetermined at fertilization. For the germ-plasm thus
contains in advance every kind of determinant in diverse _nuances_,
and in definite numerical proportions. In a plant _N´_, for instance,
_Ba´_ may be the determinant of the modern leaf-form, and may occur
in twenty-two out of twenty-four ids of the germ-plasm, while the two
remaining ids still contain unmodified the old leaf-form determinants
_Ba_, which the ancestral form _N_ possessed. But the flower of _N´_
may be of still more recent origin, and contain the modern flower
determinants _Bl´_ only in sixteen out of twenty-four ids, while in the
other eight the old flower-determinant _Bl_ of the ancestral form has
persisted. Let us now suppose that another nearly related species _P´_
has, conversely, a recently changed leaf-form but a very ancient form
of flower, so that the former is represented only in sixteen ids by the
determinants of the leaf _ba´_ and the latter in twenty-two ids by the
flower-determinants _bl´_: it is obvious that when the two species are
crossed, notwithstanding the equal number of ids in the germ-plasm, the
leaves of the hybrid will resemble more closely those of the ancestral
form _N_, and the flowers those of the form _P_; it is even conceivable
that in such a case the numerically preponderating leaf-determinants
_N_, and the equally preponderating flower-determinants of _P_ may form
a close phalanx, so to speak, against the much less numerous homologous
determinants of the other species, and that against this power working
in a definite direction the others can make no headway and are simply
condemned to inaction.

How we may or can picture this as occurring is a question which of
course admits only of being answered very hypothetically, and it leads
us, moreover, into the region of the fundamental phenomena of life,
with the interpretation of which we are not here concerned. For the
present we have assumed that life is a chemico-physical phenomenon, and
we have postponed the deeper explanation of it to the remote future,
that we may confine ourselves in the meantime to the solution of the
problem of inheritance on the basis of the forces resident in the vital
elements. But we may, nevertheless, make the supposition that a kind of
struggle between the different kinds of biophors may take place within
the cell, if the homologous determinants of all the ids for the control
of the cell have entered into it.

In many cases this struggle will be decided by the numerical
preponderance of one kind of determinant over the other, but it is
certainly conceivable that dynamic differences may also have something
to do with it.

Let us, however, abstain from trying to penetrate further into the
obscurity of these processes, and let us content ourselves with
establishing that the preponderance of one parent in some or many
parts of the child may be almost if not quite complete, and that this
compels us to assume that the hereditary substance of the other parent
is in such cases rendered inoperative--for we know it is present--since
the ids of both parents all go through the whole ontogeny, and are
contained in every somatic cell.

Upon this struggle between homologous determinants depends the
possibility of the entire suppression or inhibition of the influence
of one parent, the whole diversity in the mingling of paternal and
maternal character in the body of the offspring. It is in this that we
must seek the explanation of the fact that not only whole bodily parts
of the child, such as arms, legs, the nature of the skin, the form of
the skull, may take after sometimes the father, sometimes the mother
wholly or predominantly, but that the small separate subdivisions of
a complex organ may sometimes turn out more maternal, sometimes more
paternal. Thus intelligence from the mother and will from the father,
musical talent from the father and a talent for drawing from the
mother, may be inherited by the same child. I do not doubt that genius
depends in great part on a happy combination of such mental endowments
of the ancestors in one child. Of course something more is necessary,
namely, the strengthening of certain of these hereditary endowments,
but of this we shall speak later.

It is, however, not only the immediate ancestors, that is to say,
the parents, that have to be taken account of in this mingling of
hereditary contributions, but also those more remote. Not a few
characters in the child do not occur in either parent, but were present
in the grandparents, and their reappearance is called 'atavism' or
'reversion.' Let us consider this phenomenon in more detail, and try
to find out whether and how far it can be interpreted by means of our
theory.

The simplest and clearest cases are again found among plant-hybrids.
It may happen, for instance, that a hybrid between two species, when
dusted with its own pollen, gives rise to descendants, some of which
resemble only one of the ancestral forms: thus we have reversion to
one of the grandparents. The explanation of this lies in the different
modes in which the reducing divisions are effected; if they take place
in such a manner that all the paternal ids of the hybrid are separated
from the maternal ids, then the result is germ-cells which are like
those of the grandparents, that is, those of the parent species, and
these, if they happen to combine in amphimixis, must give rise to a
pure seedling of one or other of the two ancestral species. This case
occurs less rarely than was formerly supposed, and than it could do if
absolutely free combination of the idants took place at reduction. If
combination were quite unrestricted, all other possible combinations
would be likely to occur as frequently as these. But recent experiments
have shown that, in many plant-hybrids, the germ-cells of the hybrids
which are fertilized by their own pollen are either purely paternal or
purely maternal. There cannot, therefore, be free combination of the
idants at the reducing divisions; the idants of the two parent-forms
separate from one another and do not combine. It is doubtful, however,
whether the same thing occurs within a race, for instance, in the case
of reproduction within a human race.

In Man reversion to a grandparent occurs not infrequently, and we
may explain it thus: the id-group which controlled the type of the
grandfather was also contained in the germ-cell which gave rise to
the existence of the father, but it did not dominate the type in
that case because a more powerful id-group was opposed to it in the
germ-cell of the grandmother. When, later on, at the reducing divisions
of the germ-cells of the father, this id-group again arrived in the
sperm-cells of the father, it would predominately control the type of
the child, that is, of the third generation, provided that the egg-cell
with which it combined contained a weaker id-group.

In the case of ordinary plant-hybrids what are designated reversions
can only be called so in a wide sense, for the ancestral characters
are contained _visibly_ in the parent, although mingled with those
of the other parent. In human families, however, there are undoubted
cases in which one or more characters of the grandparent reappear in
the child which were not in any visible way expressed in the parent,
and must therefore have been contained in the parent's germ-plasm in
a latent state. And there are both in animals and plants reversions
to ancestors lying much further back, to characters and groups of
characters which have not been visible for many generations, and the
occurrence of these can only be explained on the assumption that
certain groups of ancestral determinants have been carried on in the
germ-plasm in too small a number to be able to give rise ordinarily
to the relevant character. Such isolated determinants may, however,
in certain circumstances be strengthened by the amphimixis of two
germ-cells both containing small groups of them, and thus augmented
they may gain a controlling influence. In this case the chances of the
reducing divisions have a part to play, since they bring together the
old unvaried ancestral determinants which, as we have seen, may persist
in the germ-plasm of any species through a long series of generations.
This would, of course, only suffice to bring about a reversion if the
determinants of the ancestral species were still contained in the
germ-plasm in comparative abundance. If this is no longer the case,
something more is necessary, and that is the relative weakness of the
more modern determinants.

If two white-flowered species of thorn-apple, _Datura ferox_ and
_Datura lævis_, be crossed, there arises a hybrid with bluish violet
flowers and brown stalks instead of green. This was interpreted by
Darwin as a reversion to violet-flowering ancestral species on both
sides, for there are even now a great number of species of _Datura_
with violet flowers and brown stalks. When the two white forms are
crossed the reversion takes place every time, not merely in some cases,
and we may conclude from this that in both these species there is still
such a strong admixture of the same unvaried ancestral ids that they
always excel the ids of the two modern species crossed, in strength
though certainly not in number. And this superiority must again depend
on the fact that similar determinants of the same part are cumulative
in their effect, while dissimilars are not.

For this reason reversions to remote ancestors occur readily when
species and breeds are crossed, while they are rare in the normal
inbreeding of a species. The reversions of the breeds of pigeon to
their wild ancestral form, the slaty-blue rock-pigeon, never result, as
Darwin showed, and as we have already noticed, from pure breeding of
one race, but only when two or more breeds are repeatedly crossed with
one another. Even then it occurs by no means always, but only now and
again. The germ-plasm of the breeds must therefore still contain ids
of the rock-pigeon, but in a small number, varying from individual to
individual. If by fortunate reducing divisions and the meeting together
of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell
the number of ancestral ids be raised to such a point that it exceeds
the number of modern-breed ids contained in each one of the conjugating
germ-cells, the ancestral ids control the development and reversion
occurs, for the ancestral ids together have a cumulative effect while
the ids of the two parent breeds are different and therefore, as far
as they are so, cannot be co-operative in their influence. But it must
be understood that they need not be different as far as _all_ their
determinants are concerned, but usually only as regards some groups,
and thus it happens that reversion does not occur in regard to all, but
only in regard to particular characters--thus in the _Datura_ hybrids,
chiefly in regard to the colour of the flowers and the stem, and in
the hybrids between different breeds of pigeon mainly in regard to the
colour and marking of the plumage.

The reversions of the horse and ass to striped ancestors, which Darwin
has made famous, go much further back into the ancestral history of the
species, for while we know the ancestral form of the domestic pigeon
in the still living rock-dove (_Columba livia_), the ancestral form
common to the horse and the ass is extinct, and we can only suppose
that it was striped like a zebra, because such striping occasionally
occurs in pure horses and pure asses, at least in their youth, although
now only on the legs, and because this striping is often more marked
in the hybrid between the horse and the ass, the mule. In Italy, where
one sees hundreds of mules, the striping is not exactly frequent,
but it may occur in about two per cent., while in America it is said
to be much more frequent. The germ-plasm of the horse and the ass
must therefore contain, in varying numbers, ids whose skin-colour
determinants represent in part still unmodified ancestral characters.
When two germ-cells chance to meet in fertilization, both of which have
received, through a favourable reducing division, a relatively large
number of such ids, a relative majority of these in the fertilized
ovum is opposed to the dissimilar and therefore mutually neutralizing
homologous determinants of horse and ass, and reversion to the
ancestral form occurs.

These cases of reversion are enough to show us that the old unmodified
ancestral determinants may persist in the germ-plasm through long
series of generations. But an even deeper glimpse into the dim ancient
history of our modern species of horses is afforded by the occurrence
of three-toed horses, references to a small number of which the
palæontologist Marsh was able to discover in literature, and one of
which he was able to observe in life. Julius Caesar possessed a horse
whose three-toed feet represented a reversion to the horses of Tertiary
times, _Mesohippus_, _Miohippus_, and _Protohippus_ or _Hipparion_; for
all these genera possessed, in addition to the strong middle toe, two
weaker and shorter lateral toes.

In the germ-plasm of our modern horses there must still persist in
certain ids the determinants of the ancestral foot, which, after a long
succession of favourable reducing divisions combined with favourable
chances of fertilization, may come to be in a majority, and may thus be
able to induce a reappearance of a character which has long been hidden
under the surface of the present-day type of the species.

I do not propose to enter further on the discussion of the phenomena
of inheritance. A more detailed investigation of the phenomena of
reversion is to be found in '_The Germ-plasm_' published ten years
ago; and the discussion could not be resumed here without a critical
consideration of a relatively large series of newly acquired facts
not always harmonizing, and, as yet, not even fully available. The
year 1900 has given us the investigations of three botanists, De
Vries, Correns, and Tschermak, who have sought by experiments in
hybridization between different sorts of peas, beans, maize, and other
plants, to throw light on the phenomena of inheritance, and thus on
the actual processes which occur in the germ-plasm at the reducing
division. This led to the discovery that similar experiments had been
published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and
that these had been formulated as a law which is now called Mendel's
law. Correns showed, however, that this law, though correct in certain
cases, did not by any means hold good in all, and we must thus postpone
the working of this new material into our theory until a very much
wider basis of facts has been supplied by the botanists. There is less
to be hoped for from the zoologists in regard to this problem owing
to the almost insuperable difficulties in the way of a long series
of experiments in hybridization in animals. I myself have repeatedly
attempted experiments in this direction, and have always had to abandon
them, either because the crossing succeeded too rarely, or because the
hybrids did not reproduce among themselves, or did so defectively, or
because the distinguishing characters of the crossed breeds proved
insufficiently tenacious or diagnostic. But it would be a fine task for
zoological gardens to undertake such experiments from the point of view
of the germ-plasm theory, and their success would afford material for
the criticism of the theory, the more valuable because it is apparent
from the experiments on plants that the processes of heredity are
manifold, and are far from being uniform in different domains[10].

[10] Castle and Allen have recently published the results of
experiments in crossing white mice with grey, and these confirm
Mendel's Law.

I have assumed for my theory that the reducing division took place
according to the laws of chance, and that thus every combination of ids
occurred with equal frequency. This assumption seems to be confirmed,
by the experiments of the botanists I have mentioned, only in so far
that in the crossing of hybrids with one another every combination
of distinctive characters occurred with equal frequency. But, on the
other hand, the splitting of the germ-plasm at the reducing division
seems, as I said before, in many cases to take place in such a way that
the id-groups of the two parents are discretely separated from one
another; this was so in the stocks, peas, beans, and other hybrids.
But even if this were always the case in these, we could hardly infer
that it must be the same everywhere; we should rather expect that the
relationship of the two parents and their ids would bring into play the
finer attractions and repulsions between the ids of the germ-plasm,
and would thus determine their arrangement and grouping. Further
investigations may clear up this point; in the meantime we can only
say that already--even among hybrids--many deviations from Mendel's Law
have been established, for instance, by Bateson and Saunders (1902).

Before I conclude this lecture I should like to refer briefly to a
phenomenon which Darwin was acquainted with and sought to explain
through his theory of Pangenesis, but which at a later date was
regarded as not sufficiently authenticated to justify any attempt
at a theoretical explanation, since it seemed to contradict all our
conceptions of hereditary substance and its operations. I refer to the
phenomenon to which the botanists have given the pretty name of Xenia
(guest-gifts), and which consists in the fact that in crosses of two
different plants the characters of the male may appear not only in the
young plant but even in the seed, so that a transference of paternal
characters seems to take place from the pollen-tube to the mother, to
the 'tissue of the maternal ovary.' In heads of yellow-grained maize
(_Zea_) it is said that, after dusting with pollen from a blue-seeded
variety, blue seeds appear among the yellow, and similar observations
on other cultivated plants have been on record for more than half a
century. Thus dusting the stigma of green varieties of grape with the
pollen of a dark blue kind is said frequently to give rise to dark blue
fruits.

Darwin accepted these observations as correct, and endeavoured
to explain them as due to a migration of his 'gemmules' from the
fertilized ovum into the surrounding tissue of the mother-plant. His
explanation was not correct, we can say with confidence now, but he
was right so far, for the phenomena of Xenia do occur; they are not
illusory as most modern botanists seem inclined to believe. I myself
was at first inclined to wait for further facts in proof that the
phenomena of Xenia really occurred before attempting to bring them
into harmony with my theory, and this will not be found fault with
when it is remembered that these cases of Xenia seem to stand in
direct contradiction to the fundamental postulates of the germ-plasm
theory. For this depends essentially on a definite stable structure
of the germ-substance, which lies within the nucleus in the form of
chromosomes, and which cannot pass from one cell to another in any
other way than by cell-division and division of the nucleus; how then
could it pass from the fertilized ovum to the cells of the endosperm
which do not derive their origin from it at all, but from other cells
of the embryo-sac? In point of fact some of my opponents have cited
Xenia as an actual refutation of my theory.

[Illustration: FIG. 82. Fertilization in the Lily (_Lilium martagon_),
after Guignard. _A_, the embryo-sac before fertilization; _sy_,
synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei';
_ap_, antipodal cells. _B_, the upper part of the embryo-sac,
into which the pollen-tube (_pschl_) has penetrated with the male
sex-nucleus (♂_k_) and its centrosphere; below that is the ovum-nucleus
(♀_k_) with its (also doubled) centrosphere (_csph_). _C_, remains of
the pollen-tube (_pschl_); the two sex-nuclei are closely apposed.
Highly magnified.]

That cases of Xenia really do occur is now established by the
comprehensive and at the same time exceedingly careful experiments
recently made by C. Correns with _Zea Mais_; it is only necessary to
look through the beautiful figures with which his work is adorned to be
convinced that heads of maize whose blossoms have been dusted with the
pollen of a different kind produce more or less numerous seeds of the
paternal kind, usually mingled with those of the maternal. Thus heads
of the variety _Zea alba_ resulting from fertilization with _Z. cyanea_
exhibited a majority of white grains, but among them a smaller number
of blue; and the converse experiment, of dusting _Z. cyanea_ with
the pollen of _Z. alba_, yielded heads in which a minority of white
grains appears among a majority of blue. But it is always only the
nutritive layer surrounding the embryo--the endosperm--which exhibits
the character of the paternal species, and even the capsule surrounding
the seed shows nothing of it, but is purely maternal. Thus the heads
of different species with pale-yellow capsule, when dusted with the
pollen of _Z. rubra_, never have red seeds like those of _Z. rubra_,
but always seeds with a pale-yellow skin, while, in the converse
experiment, dusting of the red-skinned species _Z. rubra_ with pollen
from _Z. vulgata_, all the seeds are red, like those of the maternal
species, and the influence of the paternal species only shows when the
strong red skin has been removed, so that the intense yellow colour of
the endosperm, which in the pure maternal species is white, is exposed
to view. Thus the mysterious influence of the pollen never goes beyond
the endosperm, and the riddle of this influence is solved in the most
unexpected manner, indeed was solved even before Correns had securely
established the genuine occurrence of Xenia. The explanation is due
to recent disclosures in regard to the processes of fertilization in
flowering plants.

It had long been known that the pollen-tube contains not merely one
generative nucleus but two, which arise from one by division. But what
had till recently remained unknown was that not only one of these
penetrated into the embryo-sac to enter into amphimixis with the
egg-cell, but that the other also makes its way in, and there fuses
with the two nuclei which had long been designated the upper and lower
polar-nuclei (Fig. 82, op. cit.). Nawaschin and Guignard demonstrated
that these two nuclei fuse _with the second male_ nucleus; thus two
acts of fertilization are accomplished in the embryo-sac, and one of
these gives rise to the embryo, while the second becomes nothing less
than the endosperm, the nutritive layer which surrounds the embryo,
whose origin from the polar nuclei had been previously recognized.

Thus the riddle of Xenia is essentially solved. We understand how
paternal primary constituents may find their way into the endosperm,
and indeed must do so regularly; we understand also how the paternal
influence never goes beyond the endosperm. The riddle is thus not only
solved, but at the same time the view which assumes a fixed germ-plasm,
and believes it to lie in the nuclear substance of the germ-cells,
receives further confirmation, if it should need any, for if facts
which are apparently contradictory to a theory can be naturally
brought into harmony with it, this affords a stronger argument for the
correctness of the theory than the power of explaining the facts which
were used in building it up.

There is much more to be said in regard to Xenia, and I am sure
that much that is of interest will be brought to light by deeper
investigation; theoretical difficulties will still have to be overcome,
and I have already pointed out one of these in my '_Germ-plasm_,' but I
must here rest satisfied with what has been already said.

We have now passed in review and attempted to fit into the theory a
sufficiently large number of the phenomena of heredity for the purpose
of these lectures. Although, as is natural, much of this must remain
hypothetical, we may accept the following series of propositions as
well founded: there is a hereditary substance, the germ-plasm; it is
contained in very minimal quantity in the germ-cells, and there in
the chromosomes of the nucleus; it consists of primary constituents
or determinants, which in their diverse arrangement beside or upon
one another form an extremely complex structure, the id. Ids and
determinants are living vital unities. Each nucleus contains several,
often many, ids, and the number of ids varies with the species and is
constant for each. The ids of the germ-plasm of each species have had
a historical development, and are derived from the germ-plasm of the
preceding lineage of species; therefore ids can never arise anew but
only through multiplication of already existing ids.

And now, equipped with this knowledge, let us return to the point
from which we started, and inquire whether the Lamarckian principle
of evolution, the inheritance of functional modifications, must be
accepted or rejected.

LECTURE XXIII

EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY OF FUNCTIONAL
MODIFICATIONS

 Darwin's Pangenesis--Alleged proofs of functional
 inheritance--Mutilations not transmissible--Brown-Séquard's
 experiments on Epilepsy in guinea-pigs--Confusion of infection of the
 germ with inheritance, Pebrine, Syphilis, and Alcoholism--Does the
 interpretation of the facts require the assumption of the transmission
 of functional modifications?--Origin of instincts--The untaught
 pointer--Vom Rath's and Morgan's views--Attachment of the dog to his
 master--Fearlessness of sea-birds and seals on lonely islands--Flies
 and butterflies--Instincts exercised only once in the course of a
 lifetime.

As I have already said in an earlier lecture, Darwin adhered to
Lamarck's assumption of the transmission of functional adaptations,
and perhaps the easiest way to make clear the theoretical difficulties
which stand in the way of such an assumption is to show how Darwin
sought to present this principle as theoretically conceivable and
possible.

Darwin was the first to think out a theory of heredity which was worthy
of the name of theory, for it was not merely an idea hastily suggested,
but an attempt, though only in outline, at elaborating a definite
hypothesis. His theory of 'Pangenesis' assumes that cells give rise to
special gemmules which are infinitesimally minute, and of which each
cell brings forth countless hosts in the course of its existence. Each
of these gemmules can give rise to a cell similar to the one in which
it was itself produced, but it cannot do this at all times, but only
under definite circumstances, namely, when it reaches 'those cells
which precede in order of development' those that it has to give rise
to. Darwin calls this the 'elective affinity' of each gemmule for
this particular kind of cell. Thus, from the beginning of development
there arises in every cell a host of gemmules, each of which virtually
represents a specific cell. These gemmules, however, do not remain
where they originated, but migrate from their place of origin into the
blood-stream, and are carried by it in myriads to all parts of the
body. Thus they reach also the ovaries and testes and the germ-cells
lying within these, penetrate into them, and there accumulate, so that
the germ-cells, in the course of life, come to contain gemmules from
all the kinds of cells which have appeared in the organism, and, at
the same time, all the variations which any part may have undergone,
whether due to external or internal influences, or through use and
disuse.

In this manner Darwin sought to attribute to the germ-cells the power
of giving rise, in the course of their development, to the same
variations as the individual had acquired during its lifetime in
consequence of external conditions or functional influences.

I abstain from analysing the assumptions here made; their improbability
and their contradictions to established facts are so great that it
is not necessary to emphasize them; the theory shows plainly that it
is necessary to have recourse to very improbable assumptions, if an
attempt is to be made to find a theoretical basis for the transmission
of acquired (somatogenic) characters. Even when Darwin formulated
his theory of Pangenesis his assumptions were hardly reconcileable
with what was known of cell-multiplication; now they are above all
irreconcileable with the fact that the germ-substance never arises
anew, but is always derived from the preceding generation--that is,
with the continuity of the germ-plasm.

If we were now to try to think out a theoretical justification we
should require to assume that the conditions of all the parts of
the body at every moment, or at least at every period of life, were
reflected in the corresponding primary constituents of the germ-plasm
and thus in the germ-cells. But, as these primary constituents are
quite different from the parts themselves, they would require to vary
in quite a different way from that in which the finished parts had
varied; which is very like supposing that an English telegram to China
is there received in the Chinese language.

In spite of this almost insuperable theoretical obstacle, various
authors have worked out the idea that the nervous system, which
connects all parts of the body with the brain and thus also with each
other, communicates these conditions to the reproductive organs, and
that thus variations may arise in the germ-cells corresponding to those
which have taken place in remote parts of the body.

Even supposing it were proved that every germ-cell in ovary or testis
was associated with a nerve-fibre, what could be transmitted to it by
the nerves, except a stronger or weaker nerve-current? There is no such
thing as _qualitative_ differences in the current; how then could the
primary constituents of the germ be influenced by the nerve-current,
either individually or in groups, in harmony with the organs and parts
of the body corresponding to them, much less be caused to vary in a
similar manner? Or are we to imagine that a particular nerve-path leads
to every one of the countless primary constituents? Or does it make
matters more intelligible if we assume that the germ-plasm is without
primary constituents, and suppose that, after each functional variation
of a part, telegraphic notice is sent to the germ-plasm by way of the
brain as to how it has to alter its 'physico-chemical constitution,'
so that the descendants may receive some benefit from the acquired
improvement?

I am not of the number of those who believe that we already know all,
or at least nearly all, that is essential, but am rather convinced that
whole regions of phenomena are still sealed to us, and I consider it
probable that the nervous system in particular is not yet exhaustively
known to us, either in regard to its functioning or in regard to its
finest structural architecture, although I gratefully recognize the
advances in this domain that the last decades have brought about. In
any case, such assumptions as I have just indicated, or similar ones,
seem to me quite too improbable to furnish any foothold for progress.
Yet we must always remain conscious that we cannot decide as to the
possibility or impossibility of any biological process whatever from
a purely theoretical standpoint, because we can only guess at, not
discern, the fundamental nature of biological processes. At the close
of this lecture I shall return to the question of the theoretical
conceivability of an inheritance of functional adaptations; but first
of all we must consider the facts and be guided by them alone. If they
prove, or even make it seem probable, that such inheritance exists,
then it must be possible, and our task is no longer to deny it, but to
find out how it can come about.

Let us therefore investigate the question whether an inheritance
of acquired characters, that is, in the first place, of functional
adaptations, is demonstrable from experience. We shall speak later on
of the effect of climatic and similar influences in causing variation;
the case in regard to them is quite different, because they undoubtedly
affect not only the parts of the body but the germ-cells as well.

When we inquire into the facts which have been brought forward by
the modern adherents of the Lamarckian principle as proofs of the
inheritance of acquired characters in this restricted sense, we shall
find that none of them can withstand criticism.

First, there are the numerous reputed cases of the inheritance of
mutilations and losses of whole parts of the body.

It is not without interest to note here how opinion in regard to this
point has altered in the course of the debate.

At the beginning of the discussion they were all brought forward as
evidence of undoubted value for the Lamarckian principle.

At the Naturalists' Congress in Wiesbaden in 1887, kittens with only
stumps of tails were exhibited, and they were said to have inherited
this peculiarity from their mother, whose tail, it was asserted, had
been accidentally amputated. The newspapers reported that the case
excited great interest, and biologists of the standing of Rudolf
Virchow declared it to be noteworthy, and regarded it as a proof, if
all the details of it were correct. From many sides similar cases were
brought forward, intended to prove that the amputation of the tail
in cats and dogs could give rise to hereditary degeneration of this
part; even students' fencing-scars were said to have been occasionally
transmitted to their sons (happily not to the daughters); a mutilated
or torn ear-lobe in the mother was said to have given rise to deformity
of the ear in a son; an injury to a father's eye was said to have
caused complete degeneration of the eyes in his children; and deformity
of a father's thumb, due to frostbite, was said to have produced
misshapen thumbs in the children and grandchildren. A multitude of
cases of this kind are to be found in the older textbooks of physiology
by Burdach, and above all by Blumenbach, and the majority have no
more than an anecdotal value, for they are not only related without
any adequate guarantee, but even without the details indispensable to
criticism.

As far back as the eighteenth century the great philosopher Kant, and
in our own day the anatomist Wilhelm His, gave their verdict decidedly
against such allegations, and absolutely denied any inheritance of
mutilations; and now, after a decade or more of lively debate over
the pros and cons, combined with detailed anatomical investigations,
careful testing of individual cases, and experiment, we are in a
position to give a decided negative and say _there is no inheritance of
mutilations_.

Let me briefly explain how this result has been reached.

In the first place, the assertion that congenital stump-tails in dogs
and cats depended on inherited mutilation proved to be unfounded. In
none of the cases of stump-tails brought forward could it even be
proved that the tail of the relevant parent had been torn or cut off,
much less that the occurrence, in parents or grandparents, of short
tails from internal causes was excluded. At the same time anatomical
investigation of such stump-tails as occur in cats in the Isle of Man,
and in many Japanese cats, and are frequently found in the most diverse
breeds of dogs, showed that these had, in their structure, nothing
in common with the remains of a tail that had been cut off, but were
spontaneous degenerations of the whole tail, and are thus deformed
tails, not shortened ones (Bonnet).

Experiments on mice also showed that the cutting off of the tail,
even when performed on both parents, does not bring about the
slightest diminution in the length of tail in the descendants. I
have myself instituted experiments of this kind, and carried them
out through twenty-two successive generations, without any positive
result. Corroborative results of these experiments on mice have
been communicated by Ritzema Bos and, independently, by Rosenthal,
and a corresponding series of experiments on rats, which these two
investigators carried out, yielded the same negative results.

When we remember that all the cases which have been brought forward in
support of an inheritance of mutilations refer to a _single_ injury
to one parent, while, in the experiments, the same mutilation was
inflicted on both parents through numerous generations, we must regard
these experiments as a proof that all earlier statements were based
either on a fallacy or on fortuitous coincidence. This conclusion is
confirmed by all that we know otherwise of the effects of oft-repeated
mutilations, as for instance the well-known mutilations and distortions
which many peoples have practised for long, sometimes inconceivably
long, ages on their children, especially circumcision, the breaking
of the incisors, the boring of holes in lip, ear, or nose, and so
forth. No child of any of these races has ever been brought into the
world with one of these marks: they have to be re-impressed on every
generation.

The experience of breeders agrees with this, and they therefore, as
Wilckens remarks, have long regarded the non-inheritance of mutilations
as an established fact. Thus there are breeds of sheep in which, for
purely practical reasons, the tails have been curtailed quite regularly
for about a century (Kühn); but no sheep with a stump-tail has ever
been born in this breed. This is all the more important because there
are other breeds of sheep (fat-rumped sheep) in which the lack of
the tail is a breed character; it is thus not the case that there is
anything in the intrinsic nature of the tail of the sheep to prevent
it becoming rudimentary. The artificially rounded ear of fox-terriers,
too, though cut for generations, never occurs hereditarily. Mr. Postans
of Eastbourne informs me that the cocks which are to be used for
cock-fighting are docked of their combs and wattles beforehand, and
that this had been done for at least a century, but that no fighting
cock without comb and wattles has been reared. In the same way various
breeds of dog, such as the spaniels, have had their tails cut to half
their length regularly and in both sexes for more than a century, yet
in this case there is no hereditary diminution of the length of tail.
Deformed stump-tails do indeed occur in most breeds of dog, but, as I
said before, their anatomical character is quite different from that of
artificially shortened tails, moreover they may occur in breeds whose
tails have not suffered from the fashion of docking, as, for instance,
in the Dachshund.

We may therefore affirm that an inheritance of artificially produced
defects and mutilations is quite unproved, and in no way bears out the
supposed inheritance of functional changes.

This is now admitted by the great majority of the adherents of the
Lamarckian principle, and we may now regard this kind of 'proof' as
disposed of.

In addition to the above, various sets of facts have been brought
forward as proofs, and in particular the much discussed experiments of
Brown-Séquard on guinea-pigs, from which it was inferred that epilepsy
artificially induced could be transmitted. But these experiments do
not really prove anything in regard to the question at issue, because
epileptic-like convulsions may have very various causes, and these
are, for the most part, quite unknown. Since artificial epilepsy
can be induced in guinea-pigs by the most diverse injuries to the
central or peripheral parts of the nervous system, this of itself
points to the fact that it is not a question of the mere lesion of
anatomical structure, I mean, of the breaking of the continuity of
a definite part, and of its transmission. The result would, in any
case, differ according to whether certain centres of the brain, or
half the spinal cord, or the main nerve-trunks were cut through. There
must, therefore, be something more needed to produce the appearance of
epilepsy--some morbid process which may arise at different parts of
the nervous system, and be continued from them to the brain-centres.
This is corroborated by the fact that it takes at least fourteen days,
and often from six to eight weeks, for epilepsy to develop after the
operation, and that in many cases it does not develop at all. I have
made the suggestion that, during or after the operation, some kind of
pathogenic micro-organism might easily reach the wounded parts, and
there excite inflammation, which may extend centripetally to the brain.
Similar processes have been observed in connexion with lymph-vessels,
and why should they not occur in connexion with nerves?

It has been objected to this that the guinea-pig's epilepsy may be
produced by blows on the skull, and also by a destructive compression
of the _nervus ischiadicus_ through the skin, and that in both cases
the epilepsy may reappear in the following generation; and this, it
is supposed, shows that the intrusion of microbes is excluded. If
this were so beyond a doubt, and if we could exclude the possibility
that there were previously various microbes within the body, which
could only penetrate into the nervous substance after the cutting
or destruction of the neurilemma, nothing would be gained that
would in any way support the Lamarckian principle. One could only
say: Certain injuries to the nervous system give rise secondarily
in guinea-pigs to morbid phenomena like epilepsy, and all sorts of
functional disturbances of the nervous system often appear in the next
generation, including in rare cases even the phenomena of epileptic
convulsions. That this is a case of the transmission of an acquired
anatomical modification brought about by the injury is not only
unproved, but is decidedly negatived, for the injuries themselves are
never transmitted. Thus what is transmitted must be quite different
from what was acquired, for no one has ever detected in the offspring
the lesion of the nerve-trunk which was cut through in the parent, or
any other result except the disease to which the original injury gives
rise. Moreover, the inheritance of these morbid phenomena has been
again brought into dispute quite recently owing to the investigations
of such experts in nervous diseases as Sommer and Binswanger, and the
correctness of Brown-Séquard's results, which have dragged through the
literature of the subject for so long, has been emphatically denied[11].

[11] See H. E. Ziegler's report in _Zool. Centralblatt_, 1900, Nos. 12
and 13.

Clearly formulated problems, like that of the inheritance of acquired
characters, should not be confused by bringing into them phenomena
whose causes are quite unknown. What do we know of the real causes
of those central brain-irritations which give rise to the phenomena
of epilepsy? It is certain enough that there are diseases which are
acquired and are yet 'inherited,' but that has nothing to do with
the Lamarckian principle, because it is a question of _infection_
of the germ, not of a definite variation in the constitution of the
germ. We know this with certainty in regard to the so-called Pebrine,
the silkworm disease which wrought such devastation in its time; the
germs of the pebrine organism have been demonstrated _in the egg_ of
the silk-moth; they multiply, not at once but later, in the young
caterpillar, and it is the half-grown caterpillar, or even the moth,
that succumbs to the disease.

Whether in this case also the disease germs are transmitted through
the male sex-cells is not proved, as far as I am aware, but that this
can happen is shown by the transmission of syphilis from father to
child. That in this case, also, the exciting cause of the disease is a
micro-organism cannot be doubted, although it has not yet been proved.
Thus even the minute spermatozoon of Man can contain microbes, and
transmit them to the germ of a new individual.

This discussion of scientific questions ought not to be brought down
to the level of a play upon words, by bringing forward cases like the
above as evidence for the inheritance of 'acquired characters,' as was
done, for instance, by M. Nussbaum, who cited as a proof of this the
migration of the alga-cells which live in the endoderm of the green
freshwater Hydra into the ovum, which is originally colourless, and
originates in the ectoderm of the animal (Fig. 35_B_, p. 169, vol.
i). It seems to me better to make a precise distinction between the
transmission of extraneous micro-organisms through the germ-cells and
the handing on of the germ-plasm with the characters inherent in its
structure. Only the latter is inheritance in the strict scientific
sense, the former is infection of the germ.

Still less than the cases of inherited traumatic epilepsy can the
morbid constitution of the children of drunkards be regarded as a
proof of the inheritance of somatogenic characters, though this has
often been maintained. I will not lay any stress on the fact that the
allegation itself is, according to the most competent observers, such
as Dr. Thomas Morton[12], far from being established. But even if it
were quite certain that the numerous diseases of the nervous system,
amounting sometimes to mania, which are frequently observed in the
children of drunkards, were really _caused_ by the drinking of the
parents, it ought not to be overlooked that we have here to do not with
the hereditary transmission of somatic variations, but of variations
_directly_ induced in the germ-plasm of the reproductive cells, for
these are exposed to the influence of the alcohol circulating in the
blood, just as any other part of the body is. That by this means
variations in the germ-plasm can be brought about, and that these
may lead to morbid conditions in the children cannot be denied, and
ought not on _a priori_ grounds to be called in question. For we are
acquainted with many other influences--climatic, for instance--which
directly affect and cause variation in the germ-plasm. Whether this
is so in the case of drunkenness, and in what manner it comes about,
whether through direct action of the alcohol, or through infection of
the germ with some microbe, we must leave to the future to decide; the
whole question is out of place here; it can in no way help us to clear
up the problem with which we are now occupied.

[12] Morton, 'The Problem of Heredity in Reference to Inebriety,'
Proceed. Soc. for the Study of Inebriety, No. 42, Nov. 1894.

But even if there were not a trace of proof of the transmissibility of
functional modifications, that alone would not justify us in concluding
that the transmission is impossible, for many things may happen that we
are not in a position to prove at present. If it could be shown that
there was a whole group of phenomena that could not be explained in any
other way than on the hypothesis of such inheritance, then we should
be obliged to assume that it really occurred, although it was not
demonstrable, and, indeed, not even theoretically conceivable. This is
the standpoint of the adherents of the Lamarckian principle at present.

They say there are a great number of transformations which are simply
and easily explained, if we regard them as the effects of inherited
use or disuse, but which admit only of a strained explanation, and
sometimes of none at all, on the basis of natural selection, and these
are not a few isolated cases, but whole categories of them.

I will submit a few of these, and show at the same time why I cannot
regard them as convincing, even if it be the case that we are not at
present in a position to explain them without the aid of the Lamarckian
principle. But let me hasten to add that it is my belief that we can do
this, although certainly not without first giving a somewhat extended
application to the principle of selection.

It has often been maintained that the existence of animal instincts is
in itself enough to prove that the Lamarckian principle is operative.
In one of the earlier lectures I showed that at least the greater
number of instincts must have originated in purely reflex actions,
and therefore, like these actions themselves, can only be explained
through natural selection. A reflex action, such as coughing, sneezing,
shutting of the eyelids, and so on, differs from an instinctive
action in the lesser complexity and shorter duration of the series
of movements liberated by a sense-impression, and also in that it
does not require to enter into consciousness at all; but no very
precise boundary can be drawn between the two, and, in any case, both
depend, as we have already seen, on a quite analogous anatomical
basis. It is only a difference in degree whether, at the sight of a
rapidly approaching object, the muscles of the eyelids contract, and
by shutting the lids, protect the eye, or whether the fly, which we
intend to seize with our hand, is impelled by the sight of the rapidly
approaching shadow of the hand to fly quickly up. The action of the fly
may be regarded as reflex, or equally well as instinctive. But there
is also only a difference in degree, not in kind, between this simple
action and the complex and protracted behaviour of a mason-bee, the
sight of whose colony impels her to fly out and fetch clay, with it
gradually to build a neat cell, to fill this with honey, to lay an egg
in it, and finally to furnish the cell with a roof of clay. Since all
reflex mechanisms, and all the natural instincts of animals, contribute
to the maintenance of the species, and are therefore useful, their
origins must be referable to natural selection, and we have only to ask
whether they must be referred to it always, and to it alone.

It cannot be doubted that, in Man, and in the higher animals voluntary
actions which are often repeated gradually acquire the character
of instinctive actions. The individual movements pertaining to the
particular action are no longer each guided by the will, but a single
exercise of will is enough to liberate the whole complex action, such
as writing, speaking, walking, or the playing of a whole piece of
music; frequently the will-impulse may be absent altogether, and the
action be set going simply by an adequate external stimulus, as in
the case of sleep-walking, which is observed in fatigued children and
soldiers, and in somnambulists. The external stimulus is transmitted
to the proper group of muscles as unfailingly as in the case of true
instincts, and this happens not only in regard to actions which,
like walking, are essential to the life of the species, but also in
regard to those which have arisen from chance habits or exercises.
Often a short practice is sufficient to make an action in this sense
instinctive, and the complexity of the instinct-mechanism gained by
such practice is often astounding. Under some circumstances a person
may play a piece on the piano from the score, and yet be thinking
intently of other things, and be quite unconscious of what is played.
In the same way it may happen that a person dominated by violent
emotion, when trying to free himself from it by reading, may read a
whole page, line by line, without understanding in the least what has
been read. In the last case it is not directly demonstrable that the
reader has made all the complex delicate eye-movements which would be
liberated by the sight of the words, but in the case of playing, the
listeners can perceive that the piece is correctly played, and thus
that the stimulus exercised by each note on the retina of the eye
is translated into the complex muscular movement of arm and finger,
corresponding both to the pitch and the duration of the note, and to
the simultaneousness of several notes.

In all these cases it is probably not always quite new paths which
are established in the brain, but use is made of particular tracks
in the innumerable nerve-paths already existing in the nerve-cells
(neurons) which are 'more thoroughly trodden' by practice, so that
the distribution of the nerve-current takes place more easily along
them than along others[13]. This much-used metaphor does not indicate
the actual structural changes which have taken place, but it serves
at least to indicate that we have to do with material changes in the
ultimate living elements of the nerve-substance (nerve-biophors)
whether these changes be in position or in quality. Now, if such
brain-structures and mechanisms acquired through exercise in the
individual life could be transmitted, new instincts would certainly
arise in this way, and many naturalists hold this view still.

[13] This, however, is by no means intended to cast doubt on the
possibility that quite new paths may arise during the individual life,
as is made probable by the recent investigations of Apáthy, Bethe, and
others.

If the inheritance of acquired characters had already been proved in
other ways, we could not refuse to admit that it might play a part in
the higher animals in the modification and new formation of instincts.
We should then have to admit that habits can be inherited, and that
instincts actually are or may be, as they have often been said to be,
inherited habits. But to make the converse conclusion, and to infer
from the result of the brain-exercise in the individual life and its
similarity to inborn instincts that the latter also depend on inherited
exercise, and that there must therefore be inheritance of acquired
characters, is hardly admissible.

It might be all very well if there were no other explanation! But as
instincts depend on material brain-mechanisms which are variable, like
every other part of the body, and as, furthermore, they are essential
to the existence of the species, and, down to the minutest detail, are
adapted to the circumstances of life, there is no obstacle in the way
of referring their origin and transformation to processes of selection.

It has been asserted that the results of training, for instance in
dogs, can be inherited, since the untaught young pointer points at
the game, and the young sheep-dog runs round and barks at the flock
of sheep without biting them. It is, however, often forgotten that,
not only have these breeds arisen under the influence of artificial
selection by Man, but that they are even now strictly selected. My
colleague and friend, Dr. Otto vom Rath, who unhappily died all too
soon for Science, and who was not only a capable investigator, but
an experienced sportsman, told me that huntsmen distinguish very
carefully between the better and the inferior young in a litter, and
that by no means every whelp of a pair of pointers can be used for
hunting game-birds. Lloyd Morgan points out the same thing, and he is
undoubtedly a competent judge in the domain of instinct; he confirms
the statement that the pointer 'often points at the quarry, it may be a
lark's nest, without instruction,' but he says at the same time, that
the power is inborn in very varying degrees, and that, in his opinion,
selection undoubtedly plays a part.

It must not, therefore, be believed that the habit of the pointer
depends on training; it is only strengthened in each individual by
training, but it depends on an innate predisposition to creep up
to the game, and is thus a form of the hunting instinct. Man has
taken advantage of this, and has increased it, but has certainly not
ingrafted it into the breed by whipping. And something similar will be
found to be true in all cases of so-called inheritance of the effects
of training. It must not be forgotten what astounding results can
be achieved in the individual by training. The elephant is the best
example of this, for it only exceptionally breeds in captivity, and
all the thousands of 'domesticated' elephants in India are tamed wild
elephants. Yet they are as gentle and docile as the horse, which has
been domesticated for thousands of years; they perform all kinds of
tasks with the greatest patience and carefulness, in many cases without
being under constant superintendence. They are indeed animals of great
intelligence; they understand what is required of them, and they
accommodate themselves readily to new conditions of life.

The attachment of the dog to its master and to Man generally has
often been cited as a proof of the origin of a new instinct by the
inheritance of acquired habitude; but the dog is a sociable animal
even in a wild state, and by living in co-operative association with
Man it has transferred its sociable affections to him. We find exactly
the same thing in the elephant which has been caught wild and tamed.
It is particularly emphasized by those who have accompanied animal
transports in Africa that the young elephants are wild and malicious
towards the blacks who teased and maltreated them, but complaisant and
harmless towards the whites who treated them kindly. The attachment of
elephants to their keepers and to every one who shows them kindness is
familiar enough; it does not depend on a newly acquired impulse, but
on the sociable impulse inherent in the species, which, in the wild
state, causes them to live in fairly large companies, and on their
inoffensive, timid, and, we may almost say, affectionate disposition.

Of course it is easy enough to give an imaginative theoretical
interpretation of the origin of a new instinct from a newly acquired
habit. We have often heard that sailors have found the birds in distant
uninhabited islands quite free from fear; they let themselves be struck
down with cudgels without attempting to escape. The extermination of
the Dodo three centuries ago is a well-known example of this. Chun, in
his magnificent work on the German Deep Sea Expedition of 1898, has
recently communicated numerous interesting examples of the indifference
of birds towards Man when they have not learned what his presence
means: thus the sea-birds of Kerguelen, penguins, cormorants, gulls,
'kelp-pigeons' (_Chionis_), and others, behaved towards Man very much
like the tame geese of our poultry yards. Even enormous mammals like
the 'sea-elephant,' a seal with a proboscis-like prolongation of the
nose, neither attempted to escape nor showed any hostility to man, but
quietly let itself be caught. Similar tales were told by Steller in
1799, after he had been obliged to pass a winter with his sailors on an
island in the Behring Straits. The numerous gigantic sea-cows (_Rhytina
stelleri_) which lived there were so confiding that they allowed the
boat to come quite up to them, and the sailors were able to kill many
of them from time to time, using their flesh for food. But towards the
end of the winter the animals began to be shy, and, in the following
winter, when other sailors to the polar regions endeavoured to hunt
them too, it was very difficult to secure them; they had recognized
man as an enemy, and fled from him when they saw him from afar. Thus
the same individuals which had earlier carelessly allowed man to come
up to them now avoided him as an enemy. _This was not instinct, it was
a behaviour controlled by the will and founded on experience._ But it
would soon become 'instinctive' if the meeting with the enemy were
often repeated, just like the winding-up of a watch, which is often
done at a wrong time, for instance, on changing clothes during the day,
and thus without reflection. It is quite easy to conceive that if the
material brain-adaptation which causes flight without reflection at
the sight of man were transmissible, the flight-instinct might become
a congenital instinct in the species in question. But this assumption
is unfounded; for, as is shown by the case of the sea-cow, we do not
require it where the animal is of sufficient intelligence to perform by
its own discernment the action necessary to its existence. The action
may thus become 'instinctive' through exercise and imitation in the
_individual_ life, without however attaining to transmissibility.

But in many cases this is not enough, namely, in all cases in which the
degree of intelligence is not sufficiently high, or where the flight
movement must follow so rapidly that it would be too late if it had to
be regulated by the will, as, for instance, the shutting of the lids
when the eye is threatened, or the flight of the fly or the butterfly
when an enemy approaches. Both fly and butterfly would be lost in every
case if they had voluntarily to set the flight-movement going after
they became conscious of danger. If they had first of all to find
out from whom danger threatened no individual would escape an early
death, and the species would die out. But they possess an instinct
which impels them to fly up with lightning speed, and in an opposite
direction, whenever they have a visual perception of the rapid approach
of any object of whatever nature. For this reason they are difficult
to catch. I once watched the play of a cat, ordinarily very clever at
catching, as she attempted to seize a peacock-butterfly (_Vanessa Io_),
which settled several times on the ground in front of her. Quietly and
slowly she crept within springing distance, but even during the spring
the butterfly flew up just before her nose and escaped every time, and
the cat gave it up after three attempts.

In this case the beginning of the action cannot lie in a voluntary
action, for the insect cannot know what it means to be caught and
killed, and the same is true of innumerable still lower animal forms,
the hermit-crabs and the Serpulids, which withdraw with lightning
speed into their houses, and so forth. It seems to me important
theoretically, that the same action can be liberated at one time by
the will, at another by the inborn instinct-mechanism. In both cases
quite similar association-changes in the nerve-centres must lie at
the root of the animal's action, but in the first case these are
developed only in the course of the individual life by exercise, while
in the second they are inborn. In the former, they are confined to the
individual, and must be acquired in each generation by imitation of
older individuals (tradition) and by inference from experience, in the
latter they are inherited as a stable character of the species.

It has been maintained by many that the origin of instincts
through processes of selection is not conceivable, because it is
improbable that the appropriate variations in the nervous system,
which are necessary for the selective establishment of the relevant
brain-mechanism, should occur fortuitously. But this is an objection
directed against the principle of selection itself, and one which
points, I think, to an incompleteness in it, as it was understood by
Darwin and Wallace. The same objection can be made to every adaptation
of an organ through natural selection; it is always doubtful whether
the useful variations will present themselves, as long as they are
due solely to chance, as the discoverers of the selective principle
assumed. We shall attempt later to fill up this gap in the theory,
but, in the meantime, I should like to point out that the process
of selection offers the only possible explanation of the origin of
instincts, since their origin through modifications of voluntary
actions into instinctive actions, with subsequent transmission of the
instinct-mechanism due to exercise in the individual life, has been
shown to be untenable.

If any one is still unconvinced of this, I can only refer to the cases
we have already discussed of instincts which are only exercised once
in a lifetime, since, in these, the only factor that can transform
a voluntary action into an instinctive one is absent, namely, the
frequent repetition of the action. In this case, if any explanation
is to be attempted at all, it can only be through natural selection,
and as we have assumed once for all that our world does admit of
explanation, we may say, _these instincts have arisen through natural
selection_.

Even though it may be difficult to think out in detail the process of
the gradual origin of such an instinctive activity, exercised once in
a lifetime, such as that, for instance, which impels the caterpillar
to spin its intricate cocoon, which it makes only once, without ever
having seen one, and thus without being able to imitate the actions
which produce it, we must not push aside the only conceivable solution
of the problem on that account, for then we should have to renounce
all hope of a scientific interpretation of the phenomenon. We may
ask, however, whether there is not something lacking in our present
conception of natural selection, and how it comes about that useful
variations always crop up and are able to increase.

But if we must explain, through natural selection, such complex series
of actions as are necessary to the making of the cocoon of the silkworm
or of the Saturnia moth (_Saturnia carpini_), what reason have we
for not referring other instincts also to selection, even if they be
repeated several times, or often, in the course of a lifetime? It is
illogical to drag in any other factor, if this one, which has been
proved to operate, is sufficient for an explanation.

Thus, as far as instincts are concerned, there is no necessity to
make the assumption of an inheritance of functional changes, any more
than there is in regard to any purely morphological modifications. As
the instincts only exercised once show us that even very complicated
impulses may arise without any inheritance of habit, that is, without
inheritance of functional modification, so there are among purely
morphological characters not a few which, though effective, are purely
passive, which are of use to the organism only through their existence,
and not through any real activity, so that they cannot be referred
to exercise, and therefore cannot be due to the transmission of the
results of exercise. And, if this be the case, then transformations
of the most diverse parts may take place without the inheritance of
acquired characters, that is, of functional modifications, and there is
no reason for dragging in an unproved mode of inheritance to explain
a process which can quite well be explained without it. For if any
part whatever can be transformed solely through natural selection,
why, since there is general variability of all parts, should this be
confined to the passive organs alone, when the active ones are equally
variable, and equally important in the struggle for existence?

There are, indeed, many of these passive parts among animals; I need
only recall the coloration of animals, the whole set of skeletal parts,
so diversely formed, of the Arthropods, the legs, wings, antennæ,
spines, hairs, claws, and so on, none of which can be changed by the
inherited results of exercise, because they are no longer capable of
modification by exercise; they are ready before they are used; they
come into use only after they have been hardened by exposure to the
air, and are no longer plastic; they are at most capable of being
used up or mutilated. Finally, even so convinced an advocate of the
Lamarckian principle as Herbert Spencer has stated that among plants
the great majority of characters and distinctive features cannot be
explained by it, but only through the principle of selection; all the
diverse protective arrangements of individual parts, like thorns,
bristles, hairs, the felt-hairs of certain leaves, the shells of nuts,
the fat and oil in seeds, the varied arrangements for the dispersal
of seeds, and so on, all operate by their presence alone, not through
any real activity which causes them to vary, and the results of which
might be transmitted. An acacia covered all over with thorns seldom
requires to use its weapons even once, and if a hungry ruminant does
prick itself on the thorns it is only a few of these which are thus
'exercised,' the rest remain untouched.

But since all these parts have originated notwithstanding their
passivity, there must be a principle which evokes them in relation to
the necessities of the conditions of life, and this can only be natural
selection, that is, the self-regulation of variations in reference to
utility. And if there is this principle, we require no other to explain
what is already explained.

I can quite well understand, however, that many naturalists, and
especially palæontologists, find it difficult to accept this
conclusion. If we think only of those parts that actively function,
and thus change by reason of their function, being strengthened by use
and weakened and diminished in size by disuse, and if, further, we
follow these parts through the history of whole geological epochs, we
may certainly get the impression that the exercise of the parts has
directly caused their phyletic evolution. The direction prescribed by
utility in the course of the individual life and in the phylogeny is
the same, and the intra-selection, that is, the selection of tissues
within the individual animal, leads towards the same improvements
as the selection of 'persons.' Thus it appears as if the phyletic
variations followed those of the individual life, while in reality the
reverse is true; the changes arising from variations in the _germ are
primary, and they determine the course of phylogenesis_, while the
tissue-selection in the individual life only elaborates and improves,
according to the demands made upon it, the material afforded by the
primordial equipment of the germ.

The American palæontologist, Osborn, cites the case of the horse's
feet as an example in support of his view that modification brought
about by use in the individual life must be transmitted in order that
the phyletic transformations may be brought about, but this example
is perhaps the best that could be chosen to prove the contrary. He
supposes that, in every young horse, the means of locomotion are
improved at every step, so to speak, through the contact with the
ground, and I am quite willing to admit that this is so. But that only
proves that, even now, an elaboration and improvement of the equipment
which the germ affords is indispensable, as it has been at all times
and in all animals, and thus that, notwithstanding the enormous number
of generations which our modern horse has behind it, the functional
acquirements of the individual have not yet been impressed upon the
germ. Why not? Because the horse becomes perfect without this, and
there was no reason why personal selection should perfect the primary
constituents of the germ still further, since the finishing touch of
perfection through use is readily afforded by the conditions of each
individual life.

Moreover, when Osborn, Cope, and other palæontologists emphasize that,
in phyletic evolutionary series, _definite paths of evolutionary_
change are adhered to, and are not deviated from either to right or to
left, they are undoubtedly right, but the conclusion which they draw is
not justifiable, whether they assume with Nägeli that there is a power
of development, a principle of perfecting, or whether, as Osborn does,
they assume the transmission of the modifications brought about through
use in the individual life. There remains a third possibility, that
the quiet and constant evolution in a definite direction is guided by
selection, and as, in passively useful parts, that principle alone is
admissible, I see no justification for assuming it to be inoperative
in regard to those which are actively functional. All these variations
which have led up, for instance, to the modern form of the horse's foot
are useful; if they were not, they could not have been produced either
by use or by disuse in the individual life.

At the same time, here again, we are justified in inquiring whether the
assumption of 'chance' germinal variations, which we have hitherto made
with Darwin and Wallace, affords a sufficient basis for selection.
Osborn says very neatly in this connexion, 'We see with Weismann and
Galton the element of chance; but the dice appear to be loaded, and in
the long run turn "sixes" up. Here arises the question: What loads the
dice?'

Until recently we might have answered, 'external conditions'; it is
they that load the dice one-sidedly, and condition that the same
straight path of phylogenesis is adhered to, and exactly the same
direction of variations is preferred and maintained. It has to be
asked, however, whether this answer, which is certainly not absolutely
incorrect, is sufficient by itself, whether the dice are not falsified
and one-sidedly loaded in another sense, so that they always throw a
preponderating number of the useful variations. We shall attempt very
soon to solve this problem, but in the meantime I must refer to another
argument in favour of assuming the Lamarckian principle, perhaps
the most important and it may be thought the most difficult of all
to refute, the so-called co-adaptation of the parts of an organism,
that is, the fitting together of many individual organs for a common
purposeful functioning.

LECTURE XXIV

OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS ARE NOT
TRANSMITTED

 Giant stag as an example of co-adaptation or 'harmonious
 adaptation'--This occurs even in passively functioning parts--Skeleton
 of Arthropods--Stridulating organ of ants and crickets--Limbs
 of the mole-cricket--Wing-venation--Colorations which form
 mimetic pictures--Harmonious adaptations in worker-bees and
 ants--Degeneration of their wings and ovaries--The quality of food
 acts as a liberating stimulus--Vom Rath's case of drones fed with
 royal food--Transition-forms between females and workers--Wasmann's
 explanation of these--The Amazon ants--Two kinds of workers--Appendix:
 Zehnder on the case of ants--On the skeleton of Arthropods--Hering's
 interpretation of Ehrlich's Ricin experiments--Hering's position in
 regard to the transmission of functional modifications.

It was Herbert Spencer, the English philosopher, who first brought
the argument of co-adaptation into the field against my view of the
non-inheritance of functionally acquired modifications. He pointed
out that many, if not, indeed, most modifications of bodily parts, to
be effective, implied further changes, often very numerous, in other
parts, and these latter must therefore have changed _simultaneously_
with the part which was being changed under the control of natural
selection; this, however, is only conceivable as due to an inheritance
of the changes caused by use, since a simultaneous alteration of so
many parts through natural selection would be impossible. If, for
instance, the antlers of our modern stag were to grow to the size of
those of the Giant Stag of the Irish peat-bogs, which measured over
ten feet across from tip to tip, this would mean--as has already been
shown--a simultaneous thickening of the skull, and to bear the heavy
burden, a strengthening of the _ligamentum nuchæ_, of the muscles of
the neck and back, of the bones of the legs and their muscles, and,
finally, of all the nerves supplying the muscles; and how could all
this happen simultaneously with, and in exact proportion to the growth
of the antlers, if it depended--as natural selection assumes--on chance
variations of all these parts? What if the appropriately favourable
variation in one of these organs did not occur? A harmonious variation
of all the parts--bones, muscles, nerves, ligaments--which unite in a
common activity, is an inadmissible assumption, because, in many cases,
such co-operating groups of organs have in the course of evolution
developed in opposite directions. In the giraffe, for instance, the
fore-legs are longer than the hind-legs, which is the reverse of what
obtains in the majority of ruminants; in the kangaroo the hind-legs,
on the contrary, have developed to a disproportionate size, while
the fore-legs have degenerated into relatively small grasping arms.
Co-operating parts, like the fore and hind limbs, may thus follow
opposite paths of evolution; their variations need not always be
directed to the same end.

The difficulty presented by these so-called co-adaptations or
harmonious correlations cannot be denied, and we must also admit
that, if the results of exercise were inherited, the explanation of
the phenomenon would, in many cases--but not, indeed, in all--be
easy, because the adaptation of the secondarily varying parts in each
individual life would correspond exactly to the altered function of
the part, and would be transmitted to the descendants, and in them
would again be subject to such a degree of variation, according to the
principle of histonal selection, as might be conditioned by the further
progress of the primary variation. The simplicity of the explanation
is striking, if only it were at the same time correct! But there are
whole series of facts, or rather of groups of facts, which prove that
the causes of co-adaptation do not lie in the inheritance of functional
modifications, and this must be recognized, even though we may not
yet be in a position to state the causes of co-adaptation, and to say
whether natural selection suffices to explain it or not.

I must first point out that co-adaptations occur not only in _actively,
but also in passively functioning parts_. Very numerous instructive
examples are to be found among the Arthropods, whose whole skeleton
belongs to this category. It has been objected that this is not wholly
passive, but that, like the bones of vertebrates, it is stimulated by
the contraction of the muscles and incited to functional reaction,
and that it thickens at places where strong muscles are inserted, and
becomes or remains thin where it is not exposed to any strain from
the muscles. But this is not the case, for the chitinous skeleton
can only offer resistance to the muscular contractions when it is no
longer soft, as it is immediately after it is secreted. As soon as it
has become hard, it can no longer be altered, and can at most be worn
away externally by long use. The proof of this lies in the necessity
for moulting, which is indispensable to all Arthropods as long as they
continue to grow, but does not occur later. Every one who has followed
the growth of an insect or a crustacean knows well that the moultings
or ecdyses are often accompanied by great changes, and hardly ever
occur without some slight changes in the form of the body, especially
of the limbs, with their teeth, bristles, spines, and so on. These
new or transformed parts are formed before the throwing-off of the
old chitinous shell, and under its protection, and they are brought
about by an elaboration or transformation of the living soft matrix
of the skeleton, the hypodermis, which consists of cells, and is the
true skin. They must thus have arisen in the ancestors of our modern
Arthropods in the same way, that is, not by a gradual modification
_during_ use, but by a slight sudden transformation before use. The
steps in the transformation may have been very small, a bristle may
have become a little longer in the second stage of life than it was
in the first, or instead of five bristles a particular spot may bear
six in the second or third stage of life; but the variations in the
phyletic development must always be caused by germ-variations which
effect from within the variation in the relevant stage of development.
But the part which has varied can only function after it has become
firm and immodifiable.

If these circumstances be kept clearly in mind, they furnish a quite
overwhelming mass of proof against the views of the Lamarckians.

Furthermore, it is not even true that the thickest parts of the
external skeleton are those at which the muscles are inserted. The
wing-covers of beetles offer the best proof to the contrary, for
there are no muscles at all in them, yet they are, in many species,
the hardest and thickest part of the whole chitinous coat of mail.
The reason is not far to seek; they protect the wings and the soft
skin of the back, which lies concealed beneath them, and the muscles
are inserted in this!--a relation which can be explained only by its
suitability to the end, and not as due to any direct effect.

When we remember the origin--which we have just described--of the
external skeleton from the soft layer of cells underneath it, the
thickness of the chitinous skeleton, which is very different at
different places in the same animal, but always adapted to its end,
furnishes a case of co-adaptation in parts which have a purely passive
function. The thickened part cannot be due to the insertion of a
muscle, but it is always there in advance, from internal causes, so
that the muscle finds sufficient resistance. Close to it there may lie,
perhaps, the edge of a segment, and at this spot the chitinous skeleton
becomes almost suddenly thinned to a joint membrane capable of being
bent or folded, not _because_ there was no pull from the muscles at
this spot, but in order that the two segments may be connected movably.
Thus, nowhere in the whole body of the Arthropod can the adaptation
of the skeleton, in regard to thickness and power of resistance, be
regulated by function itself, but only by processes of selection
which imparted to each spot the thickness it required, in order to be
effective in its function, whether that be offering resistance to the
strain of the muscles, or giving suppleness to a joint, or affording
the necessary hardness for biting the prey, or for boring into wood or
earth, or merely for protecting the animal from external injuries.

There are, however, many individual functions of the Arthropods the
exercise of which depends on the simultaneous change of several
skeletal parts; as, for instance, many of the 'singing' or vocal
apparatuses in insects. In quite recent times such vocal organs have
been discovered in ants, in which they consist of a small striated
region on the surface of the third abdominal segment, and a sharp ridge
on the segment in front; the latter is rubbed against the former by the
movements of the two segments. Quite a similar 'stridulating organ' has
long been known in the bee-ant (_Mutilla_), and the whistling sound
produced by it is easily heard by our ears; moreover August Forel has
heard it in the large wood-ant (_Camponotus ligniperdus_), and has
described it as an 'alarm-signal,' which the animals give each other
on the approach of danger--an observation which has recently been
confirmed by Wasmann and extended by Robert Wroughton in regard to
Indian ants. All these arrangements for producing sound depend always
on two organs, of which one resembles the bow, the other the strings
of a violin; the one is of no value without the other, and they must
therefore have developed simultaneously, yet they cannot have arisen
through use, and the inheritance of the results of use, because they
are both dead chitinous parts, which are never strengthened by rubbing
against each other with the movements of the abdomen, but are rather
worn away.

[Illustration: FIG. 91 (repeated). Hind-leg of a Grasshopper
(_Stenobothrus protorma_), after Graber. _fe_, femur. _ti_, tibia.
_ta_, tarsal joints. _schr_, the stridulating ridge.]

The same is true of the chirping organs of grasshoppers, beetles,
and crickets; in all cases they consist of two different parts,
which together produce a sound, and which therefore must have arisen
simultaneously, and the origin of which cannot be referred to the
inheritance of the results of exercise, but rather to selection. It is
thus possible that co-adaptation of at least two parts may take place
even when the hypothetical Lamarckian principle is altogether excluded.

When I say that we have here a case of two parts adapted to each
other, that is, strictly speaking, understating the case, for, in the
crickets and locusts, for instance, there is a whole series of peg-like
chitinous papillæ (Fig. 86), the so-called 'bridge,' each of which
must have arisen by itself through variation of the corresponding
spot of skin. At least I can see no ground for the assumption that
the chitinous surfaces on which the 'bridge' is now placed would
necessarily, from internal reasons, have varied precisely in the line
of the bridge as it has done.

[Illustration: FIG. 102. Brush and comb on the leg of a Bee (_Nomada_).
_tib_, end of the tibia. _t^1_, first tarsal joint with the brush and
its comb (_tak_). Between these and the tibial spine (_tisp_) with its
lappet (_L_) the cross-section of an antenna (_At_) is indicated. Drawn
from a preparation by Dr. Petrunkewitsch.]

Instructive examples of the co-adaptation of several parts to a common
action in organs which are not subject to the Lamarckian principle
are afforded by the diverse arrangements for cleaning the antennæ the
bearers of the smelling-organ which are so important to the life of
insects (Fig. 102). Here even the adaptation of an indented area on
the tibia of the anterior leg to the cylindrical form of the antenna
which passes through it, is sometimes so striking (Fig. 102, _tak_)
that it might be thought that it must have arisen through a gradual
wearing out; yet this is impossible, since we have to do with hard
dead chitinous surfaces, and moreover not with a solid mass, like a
hone, which is worn down by the knife, but with a hollow, thin-walled
tube. In ants, bees, and ichneumon-flies this minute, semi-circular
indentation contains small, pointed, triangular saw-teeth, closely
set like those of a comb (_tak_), and the apparatus is made usable
by the fact that a firm spine (_tisp_), fused to the end of the
tibia, overhangs the notch and presses the antenna towards it. In
many species this spine is double, or it is furnished with a thin
comb or lappet (Fig. 102, _L_), or with rows of teeth, or with short
bristles; in short, it may be equipped in the most different ways. Not
infrequently, as in wasps of the genera _Sphex_, _Scolia_, _Ammophila_,
the spine itself is also bent in a semicircle on the surface directed
towards the notch, and this may be effected in very different ways,
either by a bending of the whole thickness of the spine, or by the
presence of a comb which is concave on its inner surface. I should
never come to an end if I were to enumerate all the remarkable details
which may be found in the two main parts of this apparatus, and which
show very clearly how essential a co-operation of the two is in
fulfilling the function of cleaning the antennæ. This fitting together
of the two main parts cannot have been brought about in accordance with
the Lamarckian principle; the adaptation must therefore have come about
in some other way.

The same thing is shown by the legs and other appendages of insects and
crustaceans, which are adapted for the most diverse functions, and the
individual sections of which must be correlated if the function is to
be possible. Let us consider only the claw structures in crustaceans
and scorpions. Here, too, it seems as if the outgrowth of the last
joint of the leg, which functions as the arm of the claw, must have
arisen as a direct effect of use, through the pressure of an object
held fast by the last joint, the movable half of the claw. Frequently,
moreover, tooth-like protuberances occur on the fixed blade of the
claw (Fig. 103). But how could these have arisen as a direct effect
of pressure, since they are always preformed during the soft state of
the appendage _before use_, and are only made use of after it is fully
hardened. The soft crustaceans, the so-called 'butter-crabs' which
have just cast their shells, creep carefully away and avoid using
their limbs until they have become hard again. Here, too, we have the
co-adaptation of two parts which vary independently, and which cannot
be affected by the Lamarckian principle.

[Illustration: FIG. 103. Claw (_Sch_) on the leg of a 'Beach-fly,' an
Amphipod Crustacean (_Orchestia_). _I_, _II_, the two first joints.
_uA_, the lower blade of the claw, a non-mobile prolongation of the
penultimate joint. _oA_, the upper blade of the claw, the movable
last joint; the tubercles and indentations of the two blades fit one
another. After F. Müller.]

But the appendages furnish more complex examples of mutual adaptation.
Thus the individual sections of the anterior leg of the mole-cricket
(_Gryllotalpa_) have varied greatly, yet quite differently, and the
whole together forms a most effective digging-tool. With it the animal
digs out the earth before it to right and to left, and to do this
it makes with both legs simultaneous outward movements, which are
otherwise quite unusual among insects, and does so with such strength
that Rösel von Rosenhof saw two bodies each weighing three pounds
pushed away in this manner. In this case four chief parts of the leg
(Fig. 104), the coxa (_cox_), the femur (_fe_), the tibia (_tib_),
and the tarsi (_tars_) are so adapted to each other in form, joints,
thickness of skeleton, and size, that they cannot have varied otherwise
than in relation to each other, but each piece has done so in an
individual manner. Most remarkable of all is the short broad tibia,
equipped with four large, hard teeth, which has to perform the digging
in the ground after the manner of a spade, while the disproportionately
thin and weak tarsal joints, the last of which bears two perfectly
straight spines instead of claws, are directed upwards, and do not
touch the ground, being no longer used for walking. Rösel supposed,
probably correctly, that they are used for cleaning the spade when it
becomes clogged up with earth, since the animal cannot clean it with
its mouth. These quite unusually formed parts of the limb cannot have
become what they are as the direct results of use, because, for one
thing, it would have been not their broad surfaces, but their narrow
edges, which would most easily cut through the earth, that would have
been directed outwards. The peculiar curving, first concave, then
convex, of the outer surface of the digging foot is exactly what is
best adapted for cutting into the earth and for the pushing aside which
follows, but it is not what it would have become if the chitin-wall had
yielded to the pressure of the earth and adapted itself to it. But,
as we are again dealing with the chitinous skeleton, there can be no
question of the direct effect of use, and, it seems to me, it must be
admitted that here we have a case of co-adaptation of at least seven
different parts, which have varied independently of each other, without
any assistance from the Lamarckian principle.

[Illustration: FIG. 104. Digging leg of the Mole-cricket
(_Gryllotalpa_). _cox_, coxa attaching the limb to the thorax. _fe_,
the short broad femur. _tib_, the tibia forming a broad spade with six
large sharp teeth. _tars_, the tarsal joints, which are turned upwards
and cannot be used in locomotion. After Rösel.]

But much more complicated cases than this might be cited, if we were in
a position to estimate exactly the functional value of the individual
parts of the wing-venation in the different insects, for it is well
known that this venation serves the systematist as a basis for the
definition of genera, especially in Lepidoptera and Hymenoptera. That
is to say, it varies from genus to genus in a characteristic manner,
obviously corresponding to the differences in the wing-form, and in
the flight itself. But, unfortunately, we are still far from being
able to make more than quite general hypotheses as to the meaning of
the lengthening and strengthening, or conversely, the degeneration or
elimination, of this or that vein. From extreme cases, however, as for
instance the rich venation in good fliers with large wings, and the
scanty venation in poor fliers with small wings, we learn at least so
much, that the degree and even the manner of venation bears a definite
relation to the function of the wing, and this we might have assumed.
But these wing-veins, in as far as they serve as a support for the
weak wing-membrane, are purely chitinous structures, skeletal parts
which are not even renewed from time to time like the skeletal parts
of the leg and many other parts of the insect. As they are laid down
at first in the pupa as soft strings of cells, so they remain, and
they only begin to be used when they are completely hardened. _They
can therefore never have been caused to vary through use in the course
of the phyletic development of species and genera_, and the Lamarckian
principle can have no part in their transformations. But if they follow
the most subtle changes, which we cannot precisely demonstrate, of the
whole wing-surface and in the mode of flight, as a man is followed by
his shadow, there must be some other principle which adapts the organ
to its function, and which is able continually to adapt the large
number of individual wing-veins in the manner most advantageous for the
general function. Here, therefore, we have a state of matters exactly
corresponding to that obtaining in the transformation of actively
functioning parts which form a system with common co-operative action,
as, for instance, in the case we first discussed, that of the stag's
antlers.

Other even more complicated examples of harmonious adaptation of
passively functioning parts are afforded by the markings of animals,
such as those of the butterfly's wing. The colours have only a passive
rôle, whether they be due to pigments alone, or to structure, or to
both combined. When the coloration of a surface undergoes adaptive
variation, this cannot be due to any action of the colour, but must
depend on adaptation through selection. Yet it is well known that there
are many butterfly-wings whose surfaces exhibit different colours and
different shades of colour on their different parts, and that in such
a way that together they form a picture, that of a leaf, a piece of
bark, a stone overgrown with lichen, an eye, and so on. In such a case
the individual colour-spots stand in a particular, indirect relation
to each other; although they are independent of each other in their
variation, they are not indifferent and due to chance, for together
they produce a common picture; this is harmonious adaptation of many
parts, where the Lamarckian principle is absolutely excluded.

It may, perhaps, be objected that this mimetic picture does not
arise all at once, but very slowly in the course of long series of
generations, and, indeed, of species. This must of course be so; the
simple beginnings are complicated and perfected through the course
of long ages. This is implied in the principle of selection as we
understand it. But does any one suppose that the gigantic antlers of
the giant-stag were developed in a few generations? In this case, too,
must not numerous races have succeeded each other before the primitive
antlers attained this enormous size? If this must be assumed there was
abundance of time for the adaptation, through _germinal_ variations, of
the secondarily varying parts, the muscles, tendons, nerves, and bones,
for all these parts function actively, and can without difficulty meet,
in the individual life, the increased claims made upon them by a slight
increase in the size of the antlers. For the certain and indubitable
consequence of exercise, of increased use, is the strengthening of the
functioning parts.

Thus the appropriate germinal variation of the secondarily varying
parts may be delayed for a little without the individual being any
the less effective, or being obliged to succumb in the struggle for
existence. I do not, however, assert for a moment that the whole
explanation of the phenomena of co-adaptation is included in this;
on the contrary, I hope soon to be able to show that we may in such
cases assume a preponderance of variational tendencies in a favourable
direction, and that there is thus an indirect connexion between
the utility of a variation and its actual occurrence. In the first
place, however, I must refer to the other group of facts which I have
indicated, which show, likewise, that the simultaneous co-adaptation
of different parts may arise in certain circumstances, although the
Lamarckian principle be excluded. These are the facts presented to us
by the sterile forms of those insects, which, like bees, termites, and
ants, live together in large societies.

Ants and bees are of special interest to us in this connexion, because
they have long been carefully watched by a number of distinguished
naturalists, and most of their vital functions have been precisely
studied. Ever since the days of 'Old Peter Huber' in Geneva there have
again and again been excellent observers who have devoted almost the
whole of their life-work and talents to the more complete study of
these wonderful animals. These insects are of interest to us here,
because, in the course of the social life, a type of individual has
arisen which diverges in structure in many parts of the body from
both the male and the female, although it is sterile and does not
reproduce, or does so in so few instances that the fact is of no moment
in considering the origin of the present bodily structure. As is well
known, these neuters, or better, workers, are, among ants and bees,
females which differ from the true females not only in their smaller
size and their infertility, but in many other points as well. Among
ants, for instance, they are absolutely wingless, and at the same time
they have a much smaller and differently formed thorax and a larger
head. But the most striking point is the difference in their instincts,
for while the females, concerned only with reproduction, pair and
lay eggs, it is the workers who feed and clean the helpless emerging
larvæ, and put them in places of safety, who carry the pupæ into the
warm sunshine, and afterwards back again to the sheltered nest, who
make this nest itself, and keep it in order, after having collected or
prepared the material for it; it is they alone who defend the colony
against the attacks of enemies, who undertake predatory expeditions,
attacking the nests of other ants, and engaging in obstinate combats
with them.

How can all these peculiarities have arisen, since the workers do not
reproduce, or do so only exceptionally, and, in any case, are incapable
of pairing, and therefore--among bees at least--only produce male
offspring? Obviously it cannot have been through the transmission of
the effects of use and disuse, since they leave no offspring to which
anything could be transmitted.

Herbert Spencer has attempted to maintain the position that the
characters of the workers of to-day already existed in the pre-social
state, that is, before the ants began to form colonies, and that,
therefore, they have not been newly evolved but only preserved. But,
even if this be conceded in regard to the care of the brood and the
building instinct, so much remains that could not have existed at that
stage, that the problem of the origin of these new characters remains
unsolved. The wings, for instance, among ants, can only have been lost
when females appeared which did not reproduce, for the pairing of ants
is associated with a nuptial flight high in the air. The wings are not
merely absent in the workers, they do not even develop in the pupæ;
they are, as Dewitz showed, present even now in the larva in the form
of imaginal disks, but from the pupa-stage onwards they degenerate, and
the segments of the thorax to which they are attached likewise appear
small and modified. A variation of the germ-plasm must therefore have
taken place, and to this is due the fact that the wing-primordia no
longer develop, and that the thorax has a different development from
what it had at the time when the animals were still fertile.

It has indeed been said that there is no need for assuming a variation
of the germ-plasm, since the degeneration of the wing might be produced
by inferior nourishment. This opinion is based on the fact that, among
bees, the workers do actually arise from female larvæ which have
received a meagre diet poor in nitrogenous elements, while the same
female larvæ supplied with an abundant diet rich in nitrogen develop
into queens.

But even though we may assume that there is a similar difference in the
mode of feeding among most ants, because the workers are considerably
smaller than the fertile females, it would be quite erroneous to
conclude that the difference between the two types rests solely on the
effect of differences in diet. The elimination of an individual organ
has never yet been determined by bad and scanty nourishment; it is the
whole animal with all its parts that degenerates and becomes small and
weakly. Often as caterpillars of different species have been placed
on starvation diet, whether for experimental purposes or to procure
very small butterflies, it has never yet happened that a single organ,
such as antenna, leg, or wing, has thereby been eliminated or caused
to degenerate. I have myself instituted many such experiments with the
maggots of the blue-bottle fly, by supplying them from their earliest
youth with just as little food as possible without actually starving
them to death, yet never have these larvæ given rise to flies in which
the wings were absent or rudimentary.

Nor did these starved flies ever exhibit degenerate ovaries; they
were always completely developed and equipped with the full number
of ovarian-tubes. It was to decide this particular point that
these experiments were instituted, for my opponents maintained
that degeneration of the ovaries was a direct result of inferior
nourishment. But that is not the case. Special investigations in regard
to ants, undertaken at my request by Miss Elizabeth Bickford, showed
that the anatomical results reached by earlier investigators, like
Adlerz and Lespès, in regard to the degeneration of the ovaries in
workers, were absolutely correct, and that the 'degeneration' consists
not merely in the fact that the ovarian-tubes and ovum-primordia remain
small, but also in a diminution of the _number_ of ovarian-tubes (Fig.
105); the workers have always fewer ovarian-tubes than the females of
the same species, and--what is of especial importance--the reduction
in the number of ovarian-tubes has been effected to a different extent
in different species of ants. In the red wood-ant (_Formica rufa_) the
workers still possess from twelve to sixteen ovarian-tubes; in the
meadow-ant (_Formica pratensis_) only eight, six, or four; in _Lasius
fuliginosus_ there are usually only two (one on either side); and in
the little turf-ant (_Tetramorium cæspitum_) there are none at all. We
have here, therefore, a phylogenetic process of degeneration, which
has reached different degrees in the different species, and has only
been completed in one (_Tetramorium_). The case stands as I previously
stated it: 'The elimination of a typical organ is not an ontogenetic
process, but a phylogenetic one,' it depends not upon 'the mere
influences of nutrition which affect the development of the individual,
but always on variations in the germ-plasm, which, to all appearance,
can only come about in the course of a long series of generations'[14].

[14] _Aeussere Einflüsse als Entwickelungsreize_, Jena, 1894.

[Illustration: FIG. 105. Ovary of a fertile Queen-Ant and ovaries of
a Worker. _Od_, oviduct. _A_, one ovary of _Myrmica lævinodis_ with
many ovarian-tubes, in each of which there is an almost ripe egg (_Ei_)
and a younger egg (_Ei´_). _B_, the ovaries of a Worker of _Lasius
fuliginosus_; each ovary has only one ovarian-tube, and no ripening
egg-cells. After Elizabeth Bickford.]

Against this proposition an observation by O. vom Rath has been
cited. According to it, three drone larvæ which had been accidentally
fed by the workers with royal food exhibited striking retrogressive
peculiarities in their sexual organs. The testes contained only
immature sperms (just before emergence from the pupa), and the
copulatory organ was entirely wanting. That a certain degree of fatty
degeneration of the testes should be caused by the 'unusual fattening'
is not surprising, but it seems to me very questionable whether the
absence of the copulatory organ can be referred to the abnormal
diet; it ought to be definitely decided, by the investigation of
numerous cases, whether some abnormal peculiarity in the constitution
of the germ-plasm in these eggs was not the true cause. Hitherto,
unfortunately, I have not been able to procure the fresh material
necessary to decide this point[15]. From all this it must be evident
that we are not justified in regarding either the absence of wings or
the degeneration of the ovaries as a direct result of the inferior
nourishment supplied to the workers in the larval state: but should
any one still have doubt on this point I may mention that, among our
indigenous ants, there are two species in which the workers are just as
large as the fertile females, and that in tropical America a species
(_Myrmecocystus megalocola_) occurs in which the workers are larger
than the true females; this must mean that they have received more food
than the females, though perhaps not the same mixture of food.

[15] Since completing my manuscript I note that the point was settled
three years ago, when Koshewnikow had the opportunity of investigating
drone-pupæ which were abnormally reared in royal cells, and therefore
fed with royal food. He found their sexual organs perfectly normal, and
agrees with me that the abnormalities in Vom Rath's case must have been
due to some other cause. (See the report by Von Adelung on the Russian
paper in _Zool. Centralblatt_, Sept. 10, 1901.)

From all the facts we have discussed we can confidently conclude that
the differences in structure, which distinguish the workers from the
true females, do not depend upon the influence, in the individual
lifetime, of a poorer diet, but upon variation in the primary
constituents of the germ; we must conceive of the germ-plasm of ants as
containing, in addition to male and female ids, special ids of workers,
in which the determinants of wing and ovary are degenerate in some
degree, while the determinants of other parts, such as the brain, are
more highly developed. The manner of feeding, however, and perhaps the
mingling with the food of a special secretion of the salivary glands,
acts as a stimulus which determines whether one kind of id or another
is to be liberated, that is, to become active and to enter on the path
of development.

A proof of this view is to be found, it seems to me, in the existence
of transition forms between workers and true females, which was first
brought to general knowledge by Forel. Perhaps it would be better to
call these 'mongrel forms' for their various parts do not maintain a
medium between the two types, but many parts follow the type of the
worker, and others that of the true female. Thus Forel twice found a
nest of the red wood-ant which contained a large number of these mixed
forms, all of which possessed the small head and large curved thorax of
the queen, but otherwise resembled the workers in size and appearance,
and also in the degeneration of the ovaries. Many of them were very
small, only 5 mm. in length, and had probably received very little
food, and, according to the theory of direct influence, these should
have been pure workers. That they possessed the head and thorax of a
queen is a proof that the characters of both forms of individual were
present in the germ-plasm as primary constituents, or indeed entire
ids. In normal circumstances only one kind of these ids would have
become active, either the worker-id or the queen-id, but in abnormal
circumstances they might both be liberated to activity simultaneously,
and then they would stamp one part of the body with the character of
a queen, another with that of a worker. Forel observed one of these
nests in two successive years, and both times found the mixed forms
in large numbers[16]. In the second year he found a great number of
newly-emerged individuals of this type. I have already inferred from
this observation that the mixed forms were probably in both years
the offspring of the same mother, and this may well have been the
case. My further conclusion, that the mixed forms must be due to some
abnormality in the constitution of the germ-plasm of the maternal eggs,
no longer appears to me so convincing as it did formerly, because, in
the interval, we have learnt, through that indefatigable investigator
of ants, Pater Wasmann, that there is another possible explanation of
these mixed forms; it, too, is based upon a hypothesis, but it is so
interesting that I must briefly outline it to you.

[16] There are different kinds of 'mixed forms' among ants, which may
owe their origin to a variety of conditions, as Forel, Wasmann, and
Emery have shown in detail.

Like Forel and myself, Pater Wasmann had supposed that the reason of
this kind of mixed form (the so-called pseudogynous worker) lay in an
abnormality of the constitution of the germ-plasm, but he now regards
it as the result of a change in the mode of rearing instituted by the
workers with respect to the constitutionally female or queen larvæ,
because there was a scarcity of workers. The hypothesis sounds very
daring, but it is well founded, at least in so far that there really
is a reason why a scarcity of females must occur at certain times in
some colonies of ants, and this might certainly determine the workers
in charge of the larvæ to feed females with worker food, so as to rear
them to render the necessary assistance.

This reason lies in the occasional presence of a parasitic beetle,
_Lomechusa strumosa_, whose larvæ, curiously enough, are cared for and
fed by the ants as though they were their own, and in return they eat
up the larvæ of the ants, often destroying them in large numbers.
Wasmann informs us that the parasitic larvæ grow up just at the time at
which the ants are rearing their workers, and it is these, therefore,
which fall victims to the Lomechusa-larvæ, and the result is that a
scarcity of young workers must soon make itself felt. The workers
seek to make this good by rearing as workers all the larvæ previously
destined for queens. But this only succeeds partially, because the
development towards true females has already begun; thus mixed forms
arise.

This explanation would be rather in the air if we did not know that,
among bees, such changes in the manner of rearing are by no means
uncommon. Indeed they occur regularly when the queen of a hive perishes
and no more 'female' eggs are in store; young worker larvæ are then fed
with royal food, and these develop into queens. There can thus be no
doubt that these insects have it in their power to liberate to activity
either the female ids or the worker ids by a specific mode of feeding,
and there is nothing contrary to reason in admitting the possibility
of an alternation of this influence in the course of development, for
something analogous occurs in regard to secondary sexual characters,
as, for instance, the appearance of male decorative colours in ducks
that have become sterile.

But this change in the mode of rearing bee-larvæ gives rise to pure
queens and not to mixed forms, and we must therefore regard it as
undecided whether Wasmann's explanation is correct in this case, and
whether an abnormality in the constitution of the germ-plasm may not be
the true cause of this or other kinds of mixed forms among ants. In any
case the 'Lomechusa hypothesis' rests upon the assumption of different
kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and
the differences between the worker and queen-ants have their cause in
this, and not directly in the kind of larval food.

If there were not different ids corresponding to the different kinds
of individuals in the germ-plasm a kind of polymorphism might indeed
have arisen in the colony through differences in nutrition, but it
could not have been of the kind we now see--that is, a sharply defined
differentiation of persons, in adaptation to their different functions.
This presupposes elements in the germ which can vary slowly and
consistently in a definite direction without causing any change in the
rest of the germ.

This state of affairs gives to the phyletic evolution of the workers
a great theoretical significance, for it proves that positive as
well as negative variations of the most diverse parts of the body,
that simultaneous and correlative variations of many parts, can take
place in the course of the phylogeny, without the co-operation of
the Lamarckian factor. I have not hitherto laid any special emphasis
upon the degree of differences occurring between workers and queens;
but I must now add that this may far exceed the degree that we are
familiar with in our common indigenous ants, both in regard to
instinct and to bodily form. Even in the red Amazon ant of Western
Switzerland, _Polyergus rufescens_, we find quite a new instinct[17],
that of carrying off the pupæ of other species of ants, not to devour,
but to introduce them to their own nest and thus secure 'slaves.'
For these workers of a strange species, which emerge in a strange
nest, naturally regard the place of their birth as their home, and
do there what instinct impels them, and what they would have done in
the nest of their parents: they feed the larvæ, fetch food, collect
building material, and so on. The domestic activity of the workers
of the master-species thus becomes superfluous, and they have ceased
to exercise it, and have now entirely lost the power of caring for
their brood, searching for food, and keeping up the nest. They have
even forgotten how to take food themselves, because they are always
fed by the 'slaves.' Forel informs us--and I have myself repeated the
experiment--that _Polyergus_ workers, which are shut up with a drop
of honey on the floor of their prison, will leave it, their favourite
food, untouched, and finally starve, unless one of their 'slaves' be
shut up with them. As soon as this happens, and the slave perceives the
honey, it partakes of it, and then the 'mistress' comes and strokes the
'slave' with her antennæ to signify her desires, whereupon the 'slave'
proceeds to feed her from its own crop.

[17] 'New' in this sense, that the instinct is not exhibited by most
worker-ants, that it did not occur in the primaeval ancestors of modern
ants. It is, however, exhibited by a number of modern forms, and even
by some German species.

But while the _Polyergus_ workers have forgotten their domestic habits,
and have even ceased to be able to recognize their food, remarkable
changes have taken place in their jaws; these have lost the blunt teeth
on the inner margin, which, in other species, serve for masticating
the food, for seizing building material, and for other domestic
occupations, and have become sharp weapons, bent in the form of a
sabre, very well suited for piercing the head of an enemy, but also
well adapted for carrying off the pupæ, because they can seize them
without doing them any injury.

No one will doubt that the predatory expeditions of the Amazon ants,
and the slave-making habit, can only have developed after the habit
of living in large companies had long existed, and this case proves
that variations of instinct, as well as of bodily structure, can