or the modified product of a cell, is a more doubtful question, even if so
wide a definition be given to the term, as to include cell-like bodies
without walls and without nuclei.[898] Professor Lionel Beale uses the term
"germinal matter" for the contents of cells, taken in this wide
acceptation, and he draws a broad distinction between germinal matter and
"formed material" or the various products of cells.[899] But the doctrine
of _omnis cellula e cellulâ_ is admitted for plants, and is a widely
prevalent belief with respect to animals.[900] Thus Virchow, the great
supporter of the cellular theory, whilst allowing that difficulties exist,
maintains that every atom of tissue is derived from cells, and these from
pre-existing cells, and these primarily from the egg, which he regards as a
great cell. That cells, still retaining the same nature, increase by
self-division or proliferation, is admitted by almost every one. But when
an organism undergoes a great change of structure during development, the
cells, which at each stage are supposed to be directly derived from
previously-existing cells, must likewise be greatly changed in nature; this
change is apparently attributed by the supporters of the cellular doctrine
to some inherent power which the cells possess, and not to any external
agency.

Another school maintains that cells and tissues of all kinds may be formed,
independently of pre-existing cells, from plastic lymph or blastema; and
this it is thought is well exhibited in the repair of wounds. As I have not
especially attended to histology, it would be presumptuous in me to express
an opinion on the two opposed doctrines. But every one appears to admit
that the body consists of a multitude of "organic units,"[901] {371} each
of which possesses its own proper attributes, and is to a certain extent
independent of all others. Hence it will be convenient to use indifferently
the terms cells or organic units or simply units.

_Variability and Inheritance._--We have seen in the twenty-second chapter
that variability is not a principle co-ordinate with life or reproduction,
but results from special causes, generally from changed conditions acting
during successive generations. Part of the fluctuating variability thus
induced is apparently due to the sexual system being easily affected by
changed conditions, so that it is often rendered impotent; and when not so
seriously affected, it often fails in its proper function of transmitting
truly the characters of the parents to the offspring. But variability is
not necessarily connected with the sexual system, as we see from the cases
of bud-variation; and although we may not be able to trace the nature of
the connexion, it is probable that many deviations of structure which
appear in sexual offspring result from changed conditions acting directly
on the organisation, independently of the reproductive organs. In some
instances we may feel sure of this, when all, or nearly all the individuals
which have been similarly exposed are similarly and definitely affected--as
in the dwarfed and otherwise changed maize brought from hot countries when
cultivated in Germany; in the change of the fleece in sheep within the
tropics; to a certain extent in the increased size and early maturity of
our highly-improved domesticated animals; in inherited gout from
intemperance; and in many other such cases. Now, as such changed conditions
do not especially affect the reproductive organs, it seems mysterious on
any ordinary view why their product, the new organic being, should be
similarly affected.

How, again, can we explain to ourselves the inherited effects of the use or
disuse of particular organs? The domesticated duck flies less and walks
more than the wild duck, and its limb-bones have become in a corresponding
manner diminished and increased in comparison with those of the wild duck.
A horse is trained to certain paces, and the colt inherits similar
consensual movements. The domesticated rabbit becomes tame from close
confinement; the dog intelligent from associating with man; the retriever
is taught to fetch and carry: and these {372} mental endowments and bodily
powers are all inherited. Nothing in the whole circuit of physiology is
more wonderful. How can the use or disuse of a particular limb or of the
brain affect a small aggregate of reproductive cells, seated in a distant
part of the body, in such a manner that the being developed from these
cells inherits the characters of either one or both parents? Even an
imperfect answer to this question would be satisfactory.

Sexual reproduction does not essentially differ, as we have seen, from
budding or self-division, and these processes graduate through the repair
of injuries into ordinary development and growth; it might therefore be
expected that every character would be as regularly transmitted by all the
methods of reproduction as by continued growth. In the chapters devoted to
inheritance it was shown that a multitude of newly-acquired characters,
whether injurious or beneficial, whether of the lowest or highest vital
importance, are often faithfully transmitted--frequently even when one
parent alone possesses some new peculiarity. It deserves especial attention
that characters appearing at any age tend to reappear at a corresponding
age. We may on the whole conclude that in all cases inheritance is the
rule, and non-inheritance the anomaly. In some instances a character is not
inherited, from the conditions of life being directly opposed to its
development; in many instances, from the conditions incessantly inducing
fresh variability, as with grafted fruit-trees and highly cultivated
flowers. In the remaining cases the failure may be attributed to reversion,
by which the child resembles its grandparents or more remote progenitors,
instead of its parents.

This principle of Reversion is the most wonderful of all the attributes of
Inheritance. It proves to us that the transmission of a character and its
development, which ordinarily go together and thus escape discrimination,
are distinct powers; and these powers in some cases are even antagonistic,
for each acts alternately in successive generations. Reversion is not a
rare event, depending on some unusual or favourable combination of
circumstances, but occurs so regularly with crossed animals and plants, and
so frequently with uncrossed breeds, that it is evidently an essential part
of the principle of inheritance. We know that {373} changed conditions have
the power of evoking long-lost characters, as in the case of some feral
animals. The act of crossing in itself possesses this power in a high
degree. What can be more wonderful than that characters, which have
disappeared during scores, or hundreds, or even thousands of generations,
should suddenly reappear perfectly developed, as in the case of pigeons and
fowls when purely bred, and especially when crossed; or as with the zebrine
stripes on dun-coloured horses, and other such cases? Many monstrosities
come under this same head, as when rudimentary organs are redeveloped, or
when an organ which we must believe was possessed by an early progenitor,
but of which not even a rudiment is left, suddenly reappears, as with the
fifth stamen in some Scrophulariaceæ. We have already seen that reversion
acts in bud-reproduction; and we know that it occasionally acts during the
growth of the same individual animal, especially, but not exclusively, when
of crossed parentage,--as in the rare cases described of individual fowls,
pigeons, cattle, and rabbits, which have reverted as they advanced in years
to the colours of one of their parents or ancestors.

We are led to believe, as formerly explained, that every character which
occasionally reappears is present in a latent form in each generation, in
nearly the same manner as in male and female animals secondary characters
of the opposite sex lie latent, ready to be evolved when the reproductive
organs are injured. This comparison of the secondary sexual characters
which are latent in both sexes, with other latent characters, is the more
appropriate from the case recorded of the Hen, which assumed some of the
masculine characters, not of her own race, but of an early progenitor; she
thus exhibited at the same time the redevelopment of latent characters of
both kinds and connected both classes. In every living creature we may feel
assured that a host of lost characters lie ready to be evolved under proper
conditions. How can we make intelligible, and connect with other facts,
this wonderful and common capacity of reversion,--this power of calling
back to life long-lost characters? {374}

PART II.

I have now enumerated the chief facts which every one would desire to
connect by some intelligible bond. This can be done, as it seems to me, if
we make the following assumptions; if the first and chief one be not
rejected, the others, from being supported by various physiological
considerations, will not appear very improbable. It is almost universally
admitted that cells, or the units of the body, propagate themselves by
self-division or proliferation, retaining the same nature, and ultimately
becoming converted into the various tissues and substances of the body. But
besides this means of increase I assume that cells, before their conversion
into completely passive or "formed material," throw off minute granules or
atoms, which circulate freely throughout the system, and when supplied with
proper nutriment multiply by self-division, subsequently becoming developed
into cells like those from which they were derived. These granules for the
sake of distinctness may be called cell-gemmules, or, as the cellular
theory is not fully established, simply gemmules. They are supposed to be
transmitted from the parents to the offspring, and are generally developed
in the generation which immediately succeeds, but are often transmitted in
a dormant state during many generations and are then developed. Their
development is supposed to depend on their union with other partially
developed cells or gemmules which precede them in the regular course of
growth. Why I use the term union, will be seen when we discuss the direct
action of pollen on the tissues of the mother-plant. Gemmules are supposed
to be thrown off by every cell or unit, not only during the adult state,
but during all the stages of development. Lastly, I assume that the
gemmules in their dormant state have a mutual affinity for each other,
leading to their aggregation either into buds or into the sexual elements.
Hence, speaking strictly, it is not the reproductive elements, nor the
buds, which generate new organisms, but the cells themselves throughout the
body. These assumptions constitute the provisional hypothesis which I have
called Pangenesis. Nearly {375} similar views have been propounded, as I
find, by other authors, more especially by Mr. Herbert Spencer;[902] but
they are here modified and amplified.

{376}

Before proceeding to show, firstly, how far these assumptions are in
themselves probable, and secondly, how far they connect and explain the
various groups of facts with which we are concerned, it may be useful to
give an illustration of the hypothesis. If one of the simplest Protozoa be
formed, as appears under the microscope, of a small mass of homogeneous
gelatinous matter, a minute atom thrown off from any part and nourished
under favourable circumstances would naturally reproduce the whole; but if
the upper and lower surfaces were to differ in texture from the central
portion, then all three parts would have to throw off atoms or gemmules,
which when aggregated by mutual affinity would form either buds or the
sexual elements. Precisely the same view may be extended to one of the
higher animals; although in this case many thousand gemmules must be thrown
off from the various parts of the body. Now, when the leg, for instance, of
a salamander is cut off, a slight crust forms over the wound, and beneath
this crust the uninjured cells or units of bone, muscle, nerves, &c., are
supposed to unite with the diffused gemmules of those cells which in the
perfect leg come next in order; and these as they become slightly developed
unite with others, and so on until a papilla of soft cellular tissue, the
"budding leg," is formed, and in time a perfect leg.[903] Thus, that
portion of the leg which had {377} been cut off, neither more nor less,
would be reproduced. If the tail or leg of a young animal had been cut off,
a young tail or leg would have been reproduced, as actually occurs with the
amputated tail of the tadpole; for gemmules of all the units which compose
the tail are diffused throughout the body at all ages. But during the adult
state the gemmules of the larval tail would remain dormant, for they would
not meet with pre-existing cells in a proper state of development with
which to unite. If from changed conditions or any other cause any part of
the body should become permanently modified, the gemmules, which are merely
minute portions of the contents of the cells forming the part, would
naturally reproduce the same modification. But gemmules previously derived
from the same part before it had undergone any change, would still be
diffused throughout the organisation, and would be transmitted from
generation to generation, so that under favourable circumstances they might
be redeveloped, and then the new modification would be for a time or for
ever lost. The aggregation of gemmules derived from every part of the body,
through their mutual affinity, would form buds, and their aggregation in
some special manner, apparently in small quantity, together probably with
the presence of gemmules of certain primordial cells, would constitute the
sexual elements. By means of these illustrations the hypothesis of
pangenesis has, I hope, been rendered intelligible.

Physiologists maintain, as we have seen, that each cell, though to a large
extent dependent on others, is likewise, to a certain extent, independent
or autonomous. I go one small step further, and assume that each cell casts
off a free gemmule, which is capable of reproducing a similar cell. There
is some analogy between this view and what we see in compound animals and
in the flower-buds on the same tree; for these are distinct individuals
capable of true or seminal reproduction, yet have parts in common and are
dependent on each other; thus {378} the tree has its bark and trunk, and
certain corals, as the Virgularia, have not only parts, but movements in
common.

The existence of free gemmules is a gratuitous assumption, yet can hardly
be considered as very improbable, seeing that cells have the power of
multiplication through the self-division of their contents. Gemmules differ
from true ovules or buds inasmuch as they are supposed to be capable of
multiplication in their undeveloped state. No one probably will object to
this capacity as improbable. The blastema within the egg has been known to
divide and give birth to two embryos; and Thuret[904] has seen the zoospore
of an alga divide itself, and both halves germinate. An atom of small-pox
matter, so minute as to be borne by the wind, must multiply itself many
thousand-fold in a person thus inoculated.[905] It has recently been
ascertained[906] that a minute portion of the mucous discharge from an
animal affected with rinderpest, if placed in the blood of a healthy ox,
increases so fast that in a short space of time "the whole mass of blood,
weighing many pounds, is infected, and every small particle of that blood
contains enough poison to give, within less than forty-eight hours, the
disease to another animal."

The retention of free and undeveloped gemmules in the same body from early
youth to old age may appear improbable, but we should remember how long
seeds lie dormant in the earth and buds in the bark of a tree. Their
transmission from generation to generation may appear still more
improbable; but here again we should remember that many rudimentary and
useless organs are transmitted and have been transmitted during an
indefinite number of generations. We shall presently see how well the
long-continued transmission of undeveloped gemmules explains many facts.

As each unit, or group of similar units throughout the body, casts off its
gemmules, and as all are contained within the smallest egg or seed, and
within each spermatozoon or pollen-grain, their number and minuteness must
be something {379} inconceivable. I shall hereafter recur to this
objection, which at first appears so formidable; but it may here be
remarked that a cod-fish has been found to produce 4,872,000 eggs, a single
Ascaris about 64,000,000 eggs, and a single Orchidaceous plant probably as
many million seeds.[907] In these several cases, the spermatozoa and
pollen-grains must exist in considerably larger numbers. Now, when we have
to deal with numbers such as these, which the human intellect cannot grasp,
there is no good reason for rejecting our present hypothesis on account of
the assumed existence of cell-gemmules a few thousand times more numerous.

The gemmules in each organism must be thoroughly diffused; nor does this
seem improbable considering their minuteness, and the steady circulation of
fluids throughout the body. So it must be with the gemmules of plants, for
with certain kinds even a minute fragment of a leaf will reproduce the
whole. But a difficulty here occurs; it would appear that with plants, and
probably with compound animals, such as corals, the gemmules do not spread
from bud to bud, but only through the tissues developed from each separate
bud. We are led to this conclusion from the stock being rarely affected by
the insertion of a bud or graft from a distinct variety. This non-diffusion
of the gemmules is still more plainly shown in the case of ferns; for Mr.
Bridgman[908] has proved that, when spores (which it should be remembered
are of the nature of buds) are taken from a monstrous part of a frond, and
others from an ordinary part, {380} each reproduces the form of the part
whence derived. But this non-diffusion of the gemmules from bud to bud may
be only apparent, depending, as we shall hereafter see, on the nature of
the first-formed cells in the buds.

The assumed elective affinity of each gemmule for that particular cell
which precedes it in the order of development is supported by many
analogies. In all ordinary cases of sexual reproduction the male and female
elements have a mutual affinity for each other: thus, it is believed that
about ten thousand species of Compositæ exist, and there can be no doubt
that if the pollen of all these species could be, simultaneously or
successively, placed on the stigma of any one species, this one would elect
with unerring certainty its own pollen. This elective capacity is all the
more wonderful, as it must have been acquired since the many species of
this great group of plants branched off from a common progenitor. On any
view of the nature of sexual reproduction, the protoplasm contained within
the ovules and within the sperm-cells (or the "spermatic force" of the
latter, if so vague a term be preferred) must act on each other by some law
of special affinity, either during or subsequently to impregnation, so that
corresponding parts alone affect each other; thus, a calf produced from a
short-horned cow by a long-horned bull has its horns and not its horny
hoofs affected by the union of the two forms, and the offspring from two
birds with differently coloured tails have their tails and not their whole
plumage affected.

The various tissues of the body plainly show, as many physiologists have
insisted,[909] an affinity for special organic substances, whether natural
or foreign to the body. We see this in the cells of the kidneys attracting
urea from the blood; in the worrara poison affecting the nerves; upas and
digitalis the muscles; the Lytta vesicatoria the kidneys; and in the
poisonous matter of many diseases, as small-pox, scarlet-fever,
hooping-cough, glanders, cancer, and hydrophobia, affecting certain
definite parts of the body or certain tissues or glands.

The affinity of various parts of the body for each other during {381} their
early development was shown in the last chapter, when discussing the
tendency to fusion in homologous parts. This affinity displays itself in
the normal fusion of organs which are separate at an early embryonic age,
and still more plainly in those marvellous cases of double monsters in
which each bone, muscle, vessel, and nerve in the one embryo, blends with
the corresponding part in the other. The affinity between homologous organs
may come into action with single parts, or with the entire individual, as
in the case of flowers or fruits which are symmetrically blended together
with all their parts doubled, but without any other trace of fusion.

It has also been assumed that the development of each gemmule depends on
its union with another cell or unit which has just commenced its
development, and which, from preceding it in order of growth, is of a
somewhat different nature. Nor is it a very improbable assumption that the
development of a gemmule is determined by its union with a cell slightly
different in nature, for abundant evidence was given in the seventeenth
chapter, showing that a slight degree of differentiation in the male and
female sexual elements favours in a marked manner their union and
subsequent development. But what determines the development of the gemmules
of the first-formed or primordial cell in the unimpregnated ovule, is
beyond conjecture.

It must also be admitted that analogy fails to guide us towards any
determination on several other points: for instance, whether cells, derived
from the same parent-cell, may, in the regular course of growth, become
developed into different structures, from absorbing peculiar kinds of
nutriment, independently of their union with distinct gemmules. We shall
appreciate this difficulty if we call to mind, what complex yet symmetrical
growths the cells of plants yield when they are inoculated by the poison of
a gall-insect. With animals various polypoid excrescences and tumours are
now generally admitted[910] to be the direct product, through
proliferation, of normal cells which have become abnormal. In the regular
growth and repair of bones, the tissues undergo, as Virchow remarks,[911] a
whole series of permutations and substitutions. "The cartilage-cells may be
{382} converted by a direct transformation into marrow-cells, and continue
as such; or they may first be converted into osseous and then into
medullary tissue; or lastly, they may first be converted into marrow and
then into bone. So variable are the permutations of these tissues, in
themselves so nearly allied, and yet in their external appearance so
completely distinct." But as these tissues thus change their nature at any
age, without any obvious change in their nutrition, we must suppose in
accordance with our hypothesis that gemmules derived from one kind of
tissue combine with the cells of another kind, and cause the successive
modifications.

It is useless to speculate at what period of development each organic unit
casts off its gemmules; for the whole subject of the development of the
various elemental tissues is as yet involved in much doubt. Some
physiologists, for instance, maintain that muscle or nerve-fibres are
developed from cells, which are afterwards nourished by their own proper
powers of absorption; whilst other physiologists deny their cellular
origin; and Beale maintains that such fibres are renovated exclusively by
the conversion of fresh germinal matter (that is the so-called nuclei) into
"formed material." However this may be, it appears probable that all
external agencies, such as changed nutrition, increased use or disuse, &c.,
which induced any permanent modification in a structure, would at the same
time or previously act on the cells, nuclei, germinal or formative matter,
from which the structures in question were developed, and consequently
would act on the gemmules or cast-off atoms.

There is another point on which it is useless to speculate, namely, whether
all gemmules are free and separate, or whether some are from the first
united into small aggregates. A feather, for instance, is a complex
structure, and, as each separate part is liable to inherited variations, I
conclude that each feather certainly generates a large number of gemmules;
but it is possible that these may be aggregated into a compound gemmule.
The same remark applies to the petals of a flower, which in some cases are
highly complex, with each ridge and hollow contrived for special purposes,
so that each part must have been separately modified, and the modifications
transmitted; consequently, separate gemmules, according to our hypothesis,
{383} must have been thrown off from each cell or part. But, as we
sometimes see half an anther or a small portion of a filament becoming
petaliform, or parts or mere stripes of the calyx assuming the colour and
texture of the corolla, it is probable that with petals the gemmules of
each cell are not aggregated together into a compound gemmule, but are
freely and separately diffused.

Having now endeavoured to show that the several foregoing assumptions are
to a certain extent supported by analogous facts, and having discussed some
of the most doubtful points, we will consider how far the hypothesis brings
under a single point of view the various cases enumerated in the First
Part. All the forms of reproduction graduate into each other and agree in
their product; for it is impossible to distinguish between organisms
produced from buds, from self-division, or from fertilised germs; such
organisms are liable to variations of the same nature and to reversion of
character; and as we now see that all the forms of reproduction depend on
the aggregation of gemmules derived from the whole body, we can understand
this general agreement. It is satisfactory to find that sexual and asexual
generation, by both of which widely different processes the same living
creature is habitually produced, are fundamentally the same.
Parthenogenesis is no longer wonderful; in fact, the wonder is that it
should not oftener occur. We see that the reproductive organs do not
actually create the sexual elements; they merely determine or permit the
aggregation of the gemmules in a special manner. These organs, together
with their accessory parts, have, however, high functions to perform; they
give to both elements a special affinity for each other, independently of
the contents of the male and female cells, as is shown in the case of
plants by the mutual reaction of the stigma and pollen-grains; they adapt
one or both elements for independent temporary existence, and for mutual
union. The contrivances for these purposes are sometimes wonderfully
complex, as with the spermatophores of the Cephalopoda. The male element
sometimes possesses attributes which, if observed in an independent animal,
would be put down to instinct guided by sense-organs, as when the {384}
spermatozoon of an insect finds its way into the minute micropyle of the
egg, or as when the antherozoids of certain algæ swim by the aid of their
ciliæ to the female plant, and force themselves into a minute orifice. In
these latter cases, however, we must believe that the male element has
acquired its powers, on the same principle with the larvæ of animals,
namely by successive modifications developed at corresponding periods of
life: we can hardly avoid in these cases looking at the male element as a
sort of premature larva, which unites, or, like one of the lower algæ,
conjugates, with the female element. What determines the aggregation of the
gemmules within the sexual organs we do not in the least know; nor do we
know why buds are formed in certain definite places, leading to the
symmetrical growth of trees and corals, nor why adventitious buds may be
formed almost anywhere, even on a petal, and frequently upon healed
wounds.[912] As soon as the gemmules have aggregated themselves,
development apparently commences, but in the case of buds is often
afterwards suspended, and in the case of the sexual elements soon ceases,
unless the elements of the opposite sexes combine; even after this has
occurred, the fertilised germ, as with seeds buried in the ground, may
remain during a lengthened period in a dormant state.

The antagonism which has long been observed,[913] though exceptions
occur,[914] between active growth and the power of sexual
reproduction--between the repair of injuries and gemmation--and with
plants, between rapid increase by buds, rhizomes, &c., and the production
of seed, is partly explained by the gemmules not existing in sufficient
numbers for both processes. {385} But this explanation hardly applies to
those plants which naturally produce a multitude of seeds, but which,
through a comparatively small increase in the number of the buds on their
rhizomes or offsets, yield few or no seed. As, however, we shall presently
see that buds probably include tissue which has already been to a certain
extent developed or differentiated, some additional organised matter will
thus have been expended.

From one of the forms of Reproduction, namely, spontaneous self-division,
we are led by insensible steps to the repair of the slightest injury; and
the existence of gemmules, derived from every cell or unit throughout the
body and everywhere diffused, explains all such cases,--even the wonderful
fact that, when the limbs of the salamander were cut off many times
successively by Spallanzani and Bonnet, they were exactly and completely
reproduced. I have heard this process compared with the recrystallisation
which occurs when the angles of a broken crystal are repaired; and the two
processes have this much in common, that in the one case the polarity of
the molecules is the efficient cause, and in the other the affinity of the
gemmules for particular nascent cells.

Pangenesis does not throw much light on Hybridism, but agrees well with
most of the ascertained facts. We may conclude from the fact of a single
spermatozoon or pollen-grain being insufficient for impregnation, that a
certain number of gemmules derived from each cell or unit are required for
the development of each part. From the occurrence of parthenogenesis, more
especially in the case of the silk-moth, in which the embryo is often
partially formed, we may also infer that the female element includes nearly
sufficient gemmules of all kinds for independent development, so that when
united with the male element the gemmules must be superabundant. Now, as a
general rule, when two species or races are crossed reciprocally, the
offspring do not differ, and this shows that both sexual elements agree in
power, in accordance with the view that they include the same gemmules.
Hybrids and mongrels are generally intermediate in character between the
two parent-forms, yet occasionally they closely resemble one parent in one
part and the other parent in another part, or even in their whole
structure: nor is this difficult to understand on {386} the admission that
the gemmules in the fertilised germ are superabundant in number, and that
those derived from one parent have some advantage in number, affinity, or
vigour over those derived from the other parent. Crossed forms sometimes
exhibit the colour or other characters of either parent in stripes or
blotches; and this may occur in the first generation, or through reversion
in succeeding bud and seminal generations, as in the several instances
given in the eleventh chapter. In these cases we must follow Naudin,[915]
and admit that the "essence" or "element" of the two species, which terms I
should translate into the gemmules, have an affinity for their own kind,
and thus separate themselves into distinct stripes or blotches; and reasons
were given, when discussing in the fifteenth chapter the incompatibility of
certain characters to unite, for believing in such mutual affinity. When
two forms are crossed, one is not rarely found to be prepotent in the
transmission of character over the other; and this we can explain only by
again assuming that the one form has some advantage in the number, vigour,
or affinity of its gemmules, except in those cases, where certain
characters are present in the one form and latent in the other. For
instance, there is a latent tendency in all pigeons to become blue, and,
when a blue pigeon is crossed with one of any other colour, the blue tint
is generally prepotent. When we consider latent characters, the explanation
of this form of prepotency will be obvious.

When one species is crossed with another it is notorious that they do not
yield the full or proper number of offspring; and we can only say on this
head that, as the development of each organism depends on such
nicely-balanced affinities between a host of gemmules and developing cells
or units, we need not feel at all surprised that the commixture of gemmules
derived from two distinct species should lead to a partial or complete
failure of development. With respect to the sterility of hybrids produced
from the union of two distinct species, it was shown in the nineteenth
chapter that this depends exclusively on the reproductive organs being
specially affected; but why these organs should be thus affected we do not
know, any more than {387} why unnatural conditions of life, though
compatible with health, should cause sterility; or why continued close
interbreeding, or the illegitimate unions of dimorphic and trimorphic
plants, induce the same result. The conclusion that the reproductive organs
alone are affected, and not the whole organisation, agrees perfectly with
the unimpaired or even increased capacity in hybrid plants for propagation
by buds; for this implies, according to our hypothesis, that the cells of
the hybrids throw off hybridised cell-gemmules, which become aggregated
into buds, but fail to become aggregated within the reproductive organs, so
as to form the sexual elements. In a similar manner many plants, when
placed under unnatural conditions, fail to produce seed, but can readily be
propagated by buds. We shall presently see that pangenesis agrees well with
the strong tendency to reversion exhibited by all crossed animals and
plants.

It was shown in the discussion on graft-hybrids that there is some reason
to believe that portions of cellular tissue taken from distinct plants
become so intimately united, as afterwards occasionally to produce crossed
or hybridised buds. If this fact were fully established, it would, by the
aid of our hypothesis, connect gemmation and sexual reproduction in the
closest manner.

Abundant evidence has been advanced proving that pollen taken from one
species or variety and applied to the stigma of another sometimes directly
affects the tissues of the mother-plant. It is probable that this occurs
with many plants during fertilisation, but can only be detected when
distinct forms are crossed. On any ordinary theory of reproduction this is
a most anomalous circumstance, for the pollen-grains are manifestly adapted
to act on the ovule, but in these cases they act on the colour, texture,
and form of the coats of the seeds, on the ovarium itself, which is a
modified leaf, and even on the calyx and upper part of the flower-peduncle.
In accordance with the hypothesis of pangenesis pollen includes gemmules,
derived from every part of the organisation, which diffuse themselves and
multiply by self-division; hence it is not surprising that gemmules within
the pollen, which are derived from the parts near the reproductive organs,
should sometimes be able to affect the same parts, whilst still undergoing
development, in the mother-plant. {388}

As, during all the stages of development, the tissues of plants consist of
cells, and as new cells are not known to be formed between, or
independently of, pre-existing cells, we must conclude that the gemmules
derived from the foreign pollen do not become developed merely in contact
with pre-existing cells, but actually penetrate the nascent cells of the
mother-plant. This process may be compared with the ordinary act of
fertilisation, during which the contents of the pollen-tubes penetrate the
closed embryonic sack within the ovule, and determine the development of
the embryo. According to this view, the cells of the mother-plant may
almost literally be said to be fertilised by the gemmules derived from the
foreign pollen. With all organisms, as we shall presently see, the cells or
organic units of the embryo during the successive stages of development may
in like manner be said to be fertilised by the gemmules of the cells, which
come next in the order of formation.

Animals, when capable of sexual reproduction, are fully developed, and it
is scarcely possible that the male element should affect the tissues of the
mother in the same direct manner as with plants; nevertheless it is certain
that her ovaria are sometimes affected by a previous impregnation, so that
the ovules subsequently fertilised by a distinct male are plainly
influenced in character; and this, as in the case of foreign pollen, is
intelligible through the diffusion, retention, and action of the gemmules
included within the spermatozoa of the previous male.

Each organism reaches maturity through a longer or shorter course of
development. The changes may be small and insensibly slow, as when a child
grows into a man, or many, abrupt, and slight, as in the metamorphoses of
certain ephemerous insects, or again few and strongly marked, as with most
other insects. Each part may be moulded within a previously existing and
corresponding part, and in this case it will appear, falsely as I believe,
to be formed from the old part; or it may be developed within a wholly
distinct part of the body, as in the extreme cases of metagenesis. An eye,
for instance, may be developed at a spot where no eye previously existed.
We have also seen that allied organic beings in the course of their
metamorphoses sometimes attain nearly the same structure after passing
{389} through widely different forms; or conversely, after passing through
nearly the same early forms, arrive at a widely different termination. In
these cases it is very difficult to believe that the early cells or units
possess the inherent power, independently of any external agent, of
producing new structures wholly different in form, position, and function.
But these cases become plain on the hypothesis of pangenesis. The organic
units, during each stage of development, throw off gemmules, which,
multiplying, are transmitted to the offspring. In the offspring, as soon as
any particular cell or unit in the proper order of development becomes
partially developed, it unites with (or to speak metaphorically is
fertilised by) the gemmule of the next succeeding cell, and so onwards.
Now, supposing that at any stage of development, certain cells or
aggregates of cells had been slightly modified by the action of some
disturbing cause, the cast-off gemmules or atoms of the cell-contents could
hardly fail to be similarly affected, and consequently would reproduce the
same modification. This process might be repeated until the structure of
the part at this particular stage of development became greatly changed,
but this would not necessarily affect other parts whether previously or
subsequently developed. In this manner we can understand the remarkable
independence of structure in the successive metamorphoses, and especially
in the successive metageneses of many animals.

The term growth ought strictly to be confined to mere increase of size, and
development to change of structure.[916] Now, a child is said to grow into
a man, and a foal into a horse, but, as in these cases there is much change
of structure, the process properly belongs to the order of development. We
have indirect evidence of this in many variations and diseases supervening
during so-called growth at a particular period, and being inherited at a
corresponding period. In the case, however, of diseases which supervene
during old age, subsequently to the ordinary period of procreation, and
which nevertheless are sometimes inherited, as occurs with brain and heart
complaints, we {390} must suppose that the organs were in fact affected at
an earlier age and threw off at this period affected gemmules; but that the
affection became visible or injurious only after the prolonged growth of
the part in the strict sense of the word. In all the changes of structure
which regularly supervene during old age, we see the effects of
deteriorated growth, and not of true development.

In the so-called process of _alternate generation_ many individuals are
generated asexually during very early or later stages of development. These
individuals may closely resemble the preceding larval form, but generally
are wonderfully dissimilar. To understand this process we must suppose that
at a certain stage of development the gemmules are multiplied at an unusual
rate, and become aggregated by mutual affinity at many centres of
attraction, or buds. These buds, it may be remarked, must include gemmules
not only of all the succeeding but likewise of all the preceding stages of
development; for when mature they have the power of transmitting by sexual
generation gemmules of all the stages, however numerous these may be. It
was shown in the First Part, at least in regard to animals, that the new
beings which are thus at any period asexually generated do not retrograde
in development--that is, they do not pass through those earlier stages,
through which the fertilised germ of the same animal has to pass; and an
explanation of this fact was attempted as far as the final or teleological
cause is concerned. We can likewise understand the proximate cause, if we
assume, and the assumption is far from improbable, that buds, like
chopped-up fragments of a hydra, are formed of tissue which has already
passed through several of the earlier stages of development; for in this
case their component cells or units would not unite with the gemmules
derived from the earlier-formed cells, but only with those which came next
in the order of development. On the other hand, we must believe that, in
the sexual elements, or probably in the female alone, gemmules of certain
primordial cells are present; and these, as soon as their development
commences, unite in due succession with the gemmules of every part of the
body, from the first to the last period of life.

The principle of the independent formation of each part, in {391} so far as
its development depends on the union of the proper gemmules with certain
nascent cells, together with the superabundance of the gemmules derived
from both parents and self-multiplied, throws light on a widely different
group of facts, which on any ordinary view of development appears very
strange. I allude to organs which are abnormally multiplied or transposed.
Thus gold-fish often have supernumerary fins placed on various parts of
their bodies. We have seen that, when the tail of a lizard is broken off, a
double tail is sometimes reproduced, and when the foot of the salamander is
divided longitudinally, additional digits are occasionally formed. When
frogs, toads, &c., are born with their limbs doubled, as sometimes occurs,
the doubling, as Gervais remarks,[917] cannot be due to the complete fusion
of two embryos, with the exception of the limbs, for the larvæ are
limbless. The same argument is applicable[918] to certain insects produced
with multiple legs or antennæ, for these are metamorphosed from apodal or
antennæless larvæ. Alphonse Milne-Edwards[919] has described the curious
case of a crustacean in which one eye-peduncle supported, instead of a
complete eye, only an imperfect cornea, out of the centre of which a
portion of an antenna was developed. A case has been recorded[920] of a man
who had during both dentitions a double tooth in place of the left second
incisor, and he inherited this peculiarity from his paternal grandfather.
Several cases are known[921] of additional teeth having been developed in
the palate, more especially with horses, and in the orbit of the eye.
Certain breeds of sheep bear a whole crowd of horns on their foreheads.
Hairs occasionally appear in strange situations, as within the ears of the
Siamese hairy family; and hairs "quite natural in structure" have been
observed "within the substance of the brain."[922] As many as five spurs
have been seen on both legs in certain Game-fowls. In the Polish fowl the
male is ornamented with a topknot of hackles {392} like those on his neck,
whilst the female has one of common feathers. In feather-footed pigeons and
fowls, feathers like those on the wing arise from the outer side of the
legs and toes. Even the elemental parts of the same feather may be
transposed; for in the Sebastopol goose, barbules are developed on the
divided filaments of the shaft.

Analogous cases are of such frequent occurrence with plants that they do
not strike us with sufficient surprise. Supernumerary petals, stamens, and
pistils, are often produced. I have seen a leaflet low down in the compound
leaf of _Vicia sativa_ converted into a tendril, and a tendril possesses
many peculiar properties, such as spontaneous movement and irritability.
The calyx sometimes assumes, either wholly or by stripes, the colour and
texture of the corolla. Stamens are so frequently converted, more or less
completely, into petals, that such cases are passed over as not deserving
notice; but as petals have special functions to perform, namely, to protect
the included organs, to attract insects, and in not a few cases to guide
their entrance by well-adapted contrivances, we can hardly account for the
conversion of stamens into petals merely by unnatural or superfluous
nourishment. Again, the edge of a petal may occasionally be found including
one of the highest products of the plant, namely the pollen; for instance,
I have seen in an Ophrys a pollen-mass with its curious structure of little
packets, united together and to the caudicle by elastic threads, formed
between the edges of an upper petal. The segments of the calyx of the
common pea have been observed partially converted into carpels, including
ovules, and with their tips converted into stigmas. Numerous analogous
facts could be given.[923]

I do not know how physiologists look at such facts as the foregoing.
According to the doctrine of pangenesis, the free and superabundant
gemmules of the transposed organs are developed in the wrong place, from
uniting with wrong cells or aggregates of cells during their nascent state;
and this would follow from a slight modification in the elective affinity
of such cells, or possibly of certain gemmules. Nor ought we to feel much
surprise at the affinities of cells and gemmules varying {393} under
domestication, when we remember the many curious cases given, in the
seventeenth chapter, of cultivated plants which absolutely refuse to be
fertilised by their own pollen or by that of the same species, but are
abundantly fertile with pollen of a distinct species; for this implies that
their sexual elective affinities--and this is the term used by
Gärtner--have been modified. As the cells of adjoining or homologous parts
will have nearly the same nature, they will be liable to acquire by
variation each other's elective affinities; and we can thus to a certain
extent understand such cases as a crowd of horns on the heads in certain
sheep, of several spurs on the leg, and of hackles on the head of the fowl,
and with the pigeon the occurrence of wing-feathers on their legs and of
membrane between their toes; for the leg is the homologue of the wing. As
all the organs of plants are homologous and spring from a common axis, it
is natural that they should be eminently liable to transposition. It ought
to be observed that when any compound part, such as an additional limb or
an antenna, springs from a false position, it is only necessary that the
few first gemmules should be wrongly attached; for these whilst developing
would attract others in due succession, as in the regrowth of an amputated
limb. When parts which are homologous and similar in structure, as the
vertebræ in snakes or the stamens in polyandrous flowers, &c., are repeated
many times in the same organism, closely allied gemmules must be extremely
numerous, as well as the points to which they ought to become united; and,
in accordance with the foregoing views, we can to a certain extent
understand Isid. Geoffroy St. Hilaire's law, namely, that parts, which are
already multiple, are extremely liable to vary in number.

The same general principles apply to the fusion of homologous parts; and
with respect to mere cohesion there is probably always some degree of
fusion, at least near the surface. When two embryos during their early
development come into close contact, as both include corresponding
gemmules, which must be in all respects almost identical in nature, it is
not surprising that some derived from one embryo and some from the other
should unite at the point of contact with a single nascent cell or
aggregate of cells, and thus give rise to a single part or organ. For
instance, two embryos might thus come to have on their {394} adjoining
sides a single symmetrical arm, which in one sense will have been formed by
the fusion of the bones, muscles, &c., belonging to the arms of both
embryos. In the case of the fish described by Lereboullet, in which a
double head was seen gradually to fuse into a single one, the same process
must have taken place, together with the absorption of all the parts which
had been already formed. These cases are exactly the reverse of those in
which a part is doubled either spontaneously or after an injury; for in the
case of doubling, the superabundant gemmules of the same part are
separately developed in union with adjoining points; whilst in the case of
fusion the gemmules derived from two homologous parts become mingled and
form a single part; or it may be that the gemmules from one of two
adjoining embryos alone become developed.

Variability often depends, as I have attempted to show, on the reproductive
organs being injuriously affected by changed conditions; and in this case
the gemmules derived from the various parts of the body are probably
aggregated in an irregular manner, some superfluous and others deficient.
Whether a superabundance of gemmules, together with fusion during
development, would lead to the increased size of any part cannot be told;
but we can see that their partial deficiency, without necessarily leading
to the entire abortion of the part, might cause considerable modifications;
for in the same manner as a plant, if its own pollen be excluded, is easily
hybridised, so, in the case of a cell, if the properly succeeding gemmules
were absent, it would probably combine easily with other and allied
gemmules. We see this in the case of imperfect nails growing on the stumps
of amputated fingers,[924] for the gemmules of the nails have manifestly
been developed at the nearest point.

In variations caused by the direct action of changed conditions, whether of
a definite or indefinite nature, as with the fleeces of sheep in hot
countries, with maize grown in cold countries, with inherited gout, &c.,
the tissues of the body, according to the doctrine of pangenesis, are
directly affected by the new conditions, and consequently throw off
modified gemmules, which are transmitted with their newly acquired
peculiarities to the offspring. On any ordinary view it is unintelligible
how changed {395} conditions, whether acting on the embryo, the young or
adult animal, can cause inherited modifications. It is equally or even more
unintelligible on any ordinary view, how the effects of the long-continued
use or disuse of any part, or of changed habits of body or mind, can be
inherited. A more perplexing problem can hardly be proposed; but on our
view we have only to suppose that certain cells become at last not only
functionally but structurally modified; and that these throw off similarly
modified gemmules. This may occur at any period of development, and the
modification will be inherited at a corresponding period; for the modified
gemmules will unite in all ordinary cases with the proper preceding cells,
and they will consequently be developed at the same period at which the
modification first arose. With respect to mental habits or instincts, we
are so profoundly ignorant on the relation between the brain and the power
of thought that we do not know whether an inveterate habit or trick induces
any change in the nervous system; but when any habit or other mental
attribute, or insanity, is inherited, we must believe that some actual
modification is transmitted;[925] and this implies, according to our
hypothesis, that gemmules derived from modified nerve-cells are transmitted
to the offspring.

It is generally, perhaps always, necessary that an organism should be
exposed during several generations to changed conditions or habits, in
order that any modification in the structure of the offspring should ensue.
This may be partly due to the changes not being at first marked enough to
catch the attention, but this explanation is insufficient; and I can
account for the fact, only by the assumption, which we shall see under the
head of reversion is strongly supported, that gemmules derived from each
cell before it had undergone the least modification are transmitted in
large numbers to successive generations, but that the gemmules derived from
the same cells after modification, naturally go on increasing under the
same favouring conditions, until at last they become sufficiently numerous
to overpower and supplant the old gemmules.

Another difficulty may be here noticed; we have seen that {396} there is an
important difference in the frequency, though not in the nature, of the
variations in plants propagated by sexual and asexual generation. As far as
variability depends on the imperfect action of the reproductive organs
under changed conditions, we can at once see why seedlings should be far
more variable than plants propagated by buds. We know that extremely slight
causes,--for instance, whether a tree has been grafted or grows on its own
stock, the position of the seeds within the capsule, and of the flowers on
the spike,--sometimes suffice to determine the variation of a plant, when
raised from seed. Now, it is probable, as explained when discussing
alternate generation, that a bud is formed of a portion of already
differentiated tissue; consequently an organism thus formed does not pass
through the earlier phases of development, and cannot be so freely exposed,
at the age when its structure would be most readily modified, to the
various causes inducing variability; but it is very doubtful whether this
is a sufficient explanation of the difficulty.

With respect to the tendency to reversion, there is a similar difference
between plants propagated from buds and seed. Many varieties, whether
originally produced from seed or buds, can be securely propagated by buds,
but generally or invariably revert by seed. So, also, hybridised plants can
be multiplied to any extent by buds, but are continually liable to
reversion by seed,--that is, to the loss of their hybrid or intermediate
character. I can offer no satisfactory explanation of this fact. Here is a
still more perplexing case: certain plants with variegated leaves, phloxes
with striped flowers, barberries with seedless fruit, can all be securely
propagated by the buds on cuttings; but the buds developed from the roots
of these cuttings almost invariably lose their character and revert to
their former condition.

Finally, we can see on the hypothesis of pangenesis that variability
depends on at least two distinct groups of causes. Firstly, on the
deficiency, superabundance, fusion, and transposition of gemmules, and on
the redevelopment of those which have long been dormant. In these cases the
gemmules themselves have undergone no modification; but the mutations in
the above respects will amply account for much fluctuating {397}
variability. Secondly, in the cases in which the organisation has been
modified by changed conditions, the increased use or disuse of parts, or
any other cause, the gemmules cast off from the modified units of the body
will be themselves modified, and, when sufficiently multiplied, will be
developed into new and changed structures.

Turning now to Inheritance: if we suppose a homogeneous gelatinous
protozoon to vary and assume a reddish colour, a minute separated atom we
aid naturally, as it grew to full size, retain the same colour; and we
should have the simplest form of inheritance.[926] Precisely the same view
may be extended to the infinitely numerous and diversified units of which
the whole body in one of the higher animals is composed; and the separated
atoms are our gemmules. We have already sufficiently discussed the
inheritance of the direct effects of changed conditions, and of increased
use or disuse of parts, and, by implication, the important principle of
inheritance at corresponding ages. These groups of facts are to a large
extent intelligible on the hypothesis of pangenesis, and on no other
hypothesis as yet advanced.

A few words must be added on the complete abortion or suppression of
organs. When a part becomes diminished by disuse prolonged during many
generations, the principle of economy of growth, as previously explained,
will tend to reduce it still further; but this will not account for the
complete or almost complete obliteration of, for instance, a minute papilla
of cellular tissue representing a pistil, or of a microscopically minute
nodule of bone representing a tooth. In certain cases of suppression not
yet completed, in which a rudiment occasionally reappears through
reversion, diffused gemmules derived from this part must, according to our
view, still exist; hence we must suppose that the cells, in union with
which the rudiment was formerly developed, in these cases fail in their
affinity for such gemmules. But in the cases of complete and final abortion
the gemmules themselves no doubt have perished; nor is this {398} in any
way improbable, for, though a vast number of active and long-dormant
gemmules are diffused and nourished in each living creature, yet there must
be some limit to their number; and it appears natural that gemmules derived
from an enfeebled and useless rudiment would be more liable to perish than
those derived from other parts which are still in full functional activity.

With respect to mutilations, it is certain that a part may be removed or
injured during many generations, and no inherited result follow; and this
is an apparent objection to the hypothesis which will occur to every one.
But, in the first place, a being can hardly be intentionally mutilated
during its early stages of growth whilst in the womb or egg; and such
mutilations, when naturally caused, would appear like congenital
deficiencies, which are occasionally inherited. In the second place,
according to our hypothesis, gemmules multiply by self-division and are
transmitted from generation to generation; so that during a long period
they would be present and ready to reproduce a part which was repeatedly
amputated. Nevertheless it appears, from the facts given in the twelfth
chapter, that in some rare cases mutilations have been inherited, but in
most of these the mutilated surface became diseased. In this case it may be
conjectured that the gemmules of the lost part were gradually all attracted
by the partially diseased surface, and thus perished. Although this would
occur in the injured individual alone, and therefore in only one parent,
yet this might suffice for the inheritance of a mutilation, on the same
principle that a hornless animal of either sex, when crossed with a perfect
animal of the opposite sex, often transmits its deficiency.

The last subject that need here be discussed, namely Reversion, rests on
the principle that transmission and development, though generally acting in
conjunction, are distinct powers; and the transmission of gemmules and
their subsequent development show us how the existence of these two
distinct powers is possible. We plainly see this distinction in the many
cases in which a grandfather transmits to his grandson, through his
daughter, characters which she does not, or cannot, possess. Why the
development of certain characters, not necessarily in any way connected
with the reproductive organs, should be confined to one sex alone--that is,
why certain cells in one sex {399} should unite with and cause the
development of certain gemmules--we do not in the least know; but it is the
common attribute of most organic beings in which the sexes are separate.

The distinction between transmission and development is likewise seen in
all ordinary cases of Reversion; but before discussing this subject it may
be advisable to say a few words on those characters which I have called
latent, and which would not be classed under Reversion in its usual sense.
Most, or perhaps all, the secondary characters, which appertain to one sex,
lie dormant in the other sex; that is, gemmules capable of development into
the secondary male sexual characters are included within the female; and
conversely female characters in the male. Why in the female, when her
ovaria become diseased or fail to act, certain masculine gemmules become
developed, we do not clearly know, any more than why when a young bull is
castrated his horns continue growing until they almost resemble those of a
cow; or why, when a stag is castrated, the gemmules derived from the
antlers of his progenitors quite fail to be developed. But in many cases,
with variable organic beings, the mutual affinities of the cells and
gemmules become modified, so that parts are transposed or multiplied; and
it would appear that a slight change in the constitution of an animal, in
connection with the state of the reproductive organs, leads to changed
affinities in the tissues of various parts of the body. Thus, when male
animals first arrive at puberty, and subsequently during each recurrent
season, certain cells or parts acquire an affinity for certain gemmules,
which become developed into the secondary masculine characters; but if the
reproductive organs be destroyed, or even temporarily disturbed by changed
conditions, these affinities are not excited. Nevertheless, the male,
before he arrives at puberty, and during the season when the species does
not breed, must include the proper gemmules in a latent state. The curious
case formerly given of a Hen which assumed the masculine characters, not of
her own breed but of a remote progenitor, illustrates the connexion between
latent sexual characters and ordinary reversion. With those animals and
plants which habitually produce several forms, as with certain butterflies
described by Mr. Wallace, in which three female forms and {400} the male
exist, or as with the trimorphic species of Lythrum and Oxalis, gemmules
capable of reproducing several widely-different forms must be latent in
each individual.

The same principle of the latency of certain characters, combined with the
transposition of organs, may be applied to those singular cases of
butterflies and other insects, in which exactly one half or one quarter of
the body resembles the male, and the other half or three quarters the
female; and when this occurs the opposite sides of the body, separated from
each other by a distinct line, sometimes differ in the most conspicuous
manner. Again, these same principles apply to the cases given in the
thirteenth chapter, in which the right and left sides of the body differ to
an extraordinary degree, as in the spiral winding of certain shells, and as
in the genus Verruca among cirripedes; for in these cases it is known that
either side indifferently may undergo the same remarkable change of
development.

Reversion, in the ordinary sense of the word, comes into action so
incessantly, that it evidently forms an essential part of the general law
of inheritance. It occurs with beings, however propagated, whether by buds
or seminal generation, and sometimes may even be observed in the same
individual as it advances in age. The tendency to reversion is often
induced by a change of conditions, and in the plainest manner by the act of
crossing. Crossed forms are generally at first nearly intermediate in
character between their two parents; but in the next generation the
offspring generally revert to one or both of their grandparents, and
occasionally to more remote ancestors. How can we account for these facts?
Each organic unit in a hybrid must throw off, according to the doctrine of
pangenesis, an abundance of hybridised gemmules, for crossed plants can be
readily and largely propagated by buds; but by the same hypothesis there
will likewise be present dormant gemmules derived from both pure
parent-forms; and as these latter retain their normal condition, they
would, it is probable, be enabled to multiply largely during the lifetime
of each hybrid. Consequently the sexual elements of a hybrid will include
both pure and hybridised gemmules; and when two hybrids pair, the
combination of pure gemmules derived from the one hybrid with the pure
gemmules of the same parts derived from the other would {401} necessarily
lead to complete reversion of character; and it is, perhaps, not too bold a
supposition that unmodified and undeteriorated gemmules of the same nature
would be especially apt to combine. Pure gemmules in combination with
hybridised gemmules would lead to partial reversion. And lastly, hybridised
gemmules derived from both parent-hybrids would simply reproduce the
original hybrid form.[927] All these cases and degrees of reversion
incessantly occur.

It was shown in the fifteenth chapter that certain characters are
antagonistic to each other or do not readily blend together; hence, when
two animals with antagonistic characters are crossed, it might well happen
that a sufficiency of gemmules in the male alone for the reproduction of
his peculiar characters, and in the female alone for the reproduction of
her peculiar characters, would not be present; and in this case dormant
gemmules derived from some remote progenitor might easily gain the
ascendency, and cause the reappearance of long-lost characters. For
instance, when black and white pigeons, or black and white fowls, are
crossed,--colours which do not readily blend,--blue plumage in the one
case, evidently derived from the rock-pigeon, and red plumage in the other
case, derived from the wild jungle-cock, occasionally reappear. With
uncrossed breeds the same result would follow, under conditions which
favoured the multiplication and development of certain dormant gemmules, as
when animals become feral and revert to their pristine character. A certain
number of gemmules being requisite for the development of each character,
as is known to be the case from several spermatozoa or pollen-grains being
necessary for fertilisation, and time favouring their multiplication, will
together account for the curious cases, insisted on by Mr. Sedgwick, of
certain diseases regularly appearing in alternate generations. This
likewise holds good, more or less strictly, with other weakly inherited
modifications. Hence, as I have heard it remarked, certain diseases appear
actually to gain strength by the intermission of a generation. The
transmission of dormant gemmules during many successive generations is
hardly in itself more improbable, as {402} previously remarked, than the
retention during many ages of rudimentary organs, or even only of a
tendency to the production of a rudiment; but there is no reason to suppose
that all dormant gemmules would be transmitted and propagated for ever.
Excessively minute and numerous as they are believed to be, an infinite
number derived, during a long course of modification and descent, from each
cell of each progenitor, could not be supported or nourished by the
organism. On the other hand, it does not seem improbable that certain
gemmules, under favourable conditions, should be retained and go on
multiplying for a longer period than others. Finally, on the views here
given, we certainly gain some clear insight into the wonderful fact that
the child may depart from the type of both its parents, and resemble its
grandparents, or ancestors removed by many generations.

_Conclusion._

The hypothesis of Pangenesis, as applied to the several great classes of
facts just discussed, no doubt is extremely complex; but so assuredly are
the facts. The assumptions, however, on which the hypothesis rests cannot
be considered as complex in any extreme degree--namely, that all organic
units, besides having the power, as is generally admitted, of growing by
self-division, throw off free and minute atoms of their contents, that is
gemmules. These multiply and aggregate themselves into buds and the sexual
elements; their development depends on their union with other nascent cells
or units; and they are capable of transmission in a dormant state to
successive generations.

In a highly organised and complex animal, the gemmules thrown off from each
different cell or unit throughout the body must be inconceivably numerous
and minute. Each unit of each part, as it changes during development, and
we know that some insects undergo at least twenty metamorphoses, must throw
off its gemmules. All organic beings, moreover, include many dormant
gemmules derived from their grandparents and more remote progenitors, but
not from all their progenitors. These almost infinitely numerous and minute
gemmules must be included in each bud, ovule, spermatozoon, and
pollen-grain. Such an admission will be declared impossible; but, as
previously {403} remarked, number and size are only relative difficulties,
and the eggs or seeds produced by certain animals or plants are so numerous
that they cannot be grasped by the intellect.

The organic particles with which the wind is tainted over miles of space by
certain offensive animals must be infinitely minute and numerous; yet they
strongly affect the olfactory nerves. An analogy more appropriate is
afforded by the contagious particles of certain diseases, which are so
minute that they float in the atmosphere and adhere to smooth paper; yet we
know how largely they increase within the human body, and how powerfully
they act. Independent organisms exist which are barely visible under the
highest powers of our recently-improved microscopes, and which probably are
fully as large as the cells or units in one of the higher animals; yet
these organisms no doubt reproduce themselves by germs of extreme
minuteness, relatively to their own minute size. Hence the difficulty,
which at first appears insurmountable, of believing in the existence of
gemmules so numerous and so small as they must be according to our
hypothesis, has really little weight.

The cells or units of the body are generally admitted by physiologists to
be autonomous, like the buds on a tree, but in a less degree. I go one step
further and assume that they throw off reproductive gemmules. Thus an
animal does not, as a whole, generate its kind through the sole agency of
the reproductive system, but each separate cell generates its kind. It has
often been said by naturalists that each cell of a plant has the actual or
potential capacity of reproducing the whole plant; but it has this power
only in virtue of containing gemmules derived from every part. If our
hypothesis be provisionally accepted, we must look at all the forms of
asexual reproduction, whether occurring at maturity or as in the case of
alternate generation during youth, as fundamentally the same, and dependent
on the mutual aggregation and multiplication of the gemmules. The regrowth
of an amputated limb or the healing of a wound is the same process
partially carried out. Sexual generation differs in some important
respects, chiefly, as it would appear, in an insufficient number of
gemmules being aggregated within the separate sexual elements, and probably
in the presence of certain primordial cells. The development of each being,
including all the {404} forms of metamorphosis and metagenesis, as well as
the so-called growth of the higher animals, in which structure changes
though not in a striking manner, depends on the presence of gemmules thrown
off at each period of life, and on their development, at a corresponding
period, in union with preceding cells. Such cells may be said to be
fertilised by the gemmules which come next in the order of development.
Thus the ordinary act of impregnation and the development of each being are
closely analogous processes. The child, strictly speaking, does not grow
into the man, but includes germs which slowly and successively become
developed and form the man. In the child, as well as in the adult, each
part generates the same part for the next generation. Inheritance must be
looked at as merely a form of growth, like the self-division of a
lowly-organised unicellular plant. Reversion depends on the transmission
from the forefather to his descendants of dormant gemmules, which
occasionally become developed under certain known or unknown conditions.
Each animal and plant may be compared to a bed of mould full of seeds, most
of which soon germinate, some lie for a period dormant, whilst others
perish. When we hear it said that a man carries in his constitution the
seeds of an inherited disease, there is much literal truth in the
expression. Finally, the power of propagation possessed by each separate
cell, using the term in its largest sense, determines the reproduction, the
variability, the development and renovation of each living organism. No
other attempt, as far as I am aware, has been made, imperfect as this
confessedly is, to connect under one point of view these several grand
classes of facts. We cannot fathom the marvellous complexity of an organic
being; but on the hypothesis here advanced this complexity is much
increased. Each living creature must be looked at as a microcosm--a little
universe, formed of a host of self-propagating organisms, inconceivably
minute and as numerous as the stars in heaven.

{405}

CHAPTER XXVIII.

CONCLUDING REMARKS.

    DOMESTICATION--NATURE AND CAUSES OF VARIABILITY--SELECTION--DIVERGENCE
    AND DISTINCTNESS OF CHARACTER--EXTINCTION OF RACES--CIRCUMSTANCES
    FAVOURABLE TO SELECTION BY MAN--ANTIQUITY OF CERTAIN RACES--THE
    QUESTION WHETHER EACH PARTICULAR VARIATION HAS BEEN SPECIALLY
    PREORDAINED.

As summaries have been added to nearly all the chapters, and as, in the
chapter on pangenesis, various subjects, such as the forms of reproduction,
inheritance, reversion, the causes and laws of variability, &c., have been
recently discussed, I will here only make a few general remarks on the more
important conclusions which may be deduced from the multifarious details
given throughout this work.

Savages in all parts of the world easily succeed in taming wild animals;
and those inhabiting any country or island, when first invaded by man,
would probably have been still more easily tamed. Complete subjugation
generally depends on an animal being social in its habits, and on receiving
man as the chief of the herd or family. Domestication implies almost
complete fertility under new and changed conditions of life, and this is
far from being invariably the case. An animal would not have been worth the
labour of domestication, at least during early times, unless of service to
man. From these circumstances the number of domesticated animals has never
been large. With respect to plants, I have shown in the ninth chapter how
their varied uses were probably first discovered, and the early steps in
their cultivation. Man could not have known, when he first domesticated an
animal or plant, whether it would flourish and multiply when transported to
other countries, therefore he could not have been thus influenced in his
choice. We see that the close adaptation of the reindeer and camel to
extremely cold and hot countries has not prevented their domestication.
Still less {406} could man have foreseen whether his animals and plants
would vary in succeeding generations and thus give birth to new races; and
the small capacity of variability in the goose and ass has not prevented
their domestication from the remotest epoch.

With extremely few exceptions, all animals and plants which have been long
domesticated, have varied greatly. It matters not under what climate, or
for what purpose, they are kept, whether as food for man or beast, for
draught or hunting, for clothing or mere pleasure,--under all these
circumstances domesticated animals and plants have varied to a much greater
extent than the forms which in a state of nature are ranked as one species.
Why certain animals and plants have varied more under domestication than
others we do not know, any more than why some are rendered more sterile
than others under changed conditions of life. But we frequently judge of
the amount of variation by the production of numerous and diversified
races, and we can clearly see why in many cases this has not occurred,
namely, because slight successive variations have not been steadily
accumulated; and such variations will never be accumulated when an animal
or plant is not closely observed, or much valued, or kept in large numbers.

The fluctuating, and, as far as we can judge, never-ending variability of
our domesticated productions,--the plasticity of their whole
organisation,--is one of the most important facts which we learn from the
numerous details given in the earlier chapters of this work. Yet
domesticated animals and plants can hardly have been exposed to greater
changes in their conditions than have many natural species during the
incessant geological, geographical, and climatal changes of the whole
world. The former will, however, commonly have been exposed to more sudden
changes and to less continuously uniform conditions. As man has
domesticated so many animals and plants belonging to widely different
classes, and as he certainly did not with prophetic instinct choose those
species which would vary most, we may infer that all natural species, if
subjected to analogous conditions, would, on an average, vary to the same
degree. Few men at the present day will maintain that animals and plants
were created with a tendency to vary, which long remained dormant, in order
that fanciers in after ages might {407} rear, for instance, curious breeds
of the fowl, pigeon, or canary-bird.

From several causes it is difficult to judge of the amount of modification
which our domestic productions have undergone. In some cases the primitive
parent-stock has become extinct, or cannot be recognised with certainty
owing to its supposed descendants having been so much modified. In other
cases two or more closely allied forms, after being domesticated, have
crossed; and then it is difficult to estimate how much of the change ought
to be attributed to variation. But the degree to which our domestic breeds
have been modified by the crossing of distinct natural forms has probably
been exaggerated by some authors. A few individuals of one form would
seldom permanently affect another form existing in much greater numbers;
for, without careful selection, the stain of the foreign blood would soon
be obliterated, and during early and barbarous times, when our animals were
first domesticated, such care would seldom have been taken.

There is good reason to believe that several of the breeds of the dog, ox,
pig, and of some other animals, are respectively descended from distinct
wild prototypes; nevertheless the belief in the multiple origin of our
domesticated animals has been extended by some few naturalists and by many
breeders to an unauthorised extent. Breeders refuse to look at the whole
subject under a single point of view; I have heard one, who maintained that
our fowls were the descendants of at least half-a-dozen aboriginal species,
protest that he was in no way concerned with the origin of pigeons, ducks,
rabbits, horses, or any other animal. They overlook the improbability of
many species having been domesticated at an early and barbarous period.
They do not consider the improbability of species having existed in a state
of nature which, if like our present domestic breeds, would have been
highly abnormal in comparison with all their congeners. They maintain that
certain species, which formerly existed, have become extinct or unknown,
although the world is now so much better known. The assumption of so much
recent extinction is no difficulty in their eyes; for they do not judge of
its probability by the facility or difficulty of the extinction of other
closely allied wild forms. Lastly, {408} they often ignore the whole
subject of geographical distribution as completely as if its laws were the
result of chance.

Although from the reasons just assigned it is often difficult to judge
accurately of the amount of change which our domesticated productions have
undergone, yet this can be ascertained in the cases in which we know that
all the breeds are descended from a single species, as with the pigeon,
duck, rabbit, and almost certainly with the fowl; and by the aid of analogy
this is to a certain extent possible in the case of animals descended from
several wild stocks. It is impossible to read the details given in the
earlier chapters, and in many published works, or to visit our various
exhibitions, without being deeply impressed with the extreme variability of
our domesticated animals and cultivated plants. I have in many instances
purposely given details on new and strange peculiarities which have arisen.
No part of the organisation escapes the tendency to vary. The variations
generally affect parts of small vital or physiological importance, but so
it is with the differences which exist between closely allied species. In
these unimportant characters there is often a greater difference between
the breeds of the same species than between the natural species of the same
genus, as Isidore Geoffroy has shown to be the case with size, and as is
often the case with the colour, texture, form, &c., of the hair, feathers,
horns, and other dermal appendages.

It has often been asserted that important parts never vary under
domestication, but this is a complete error. Look at the skull of the pig
in any one of the highly improved breeds, with the occipital condyles and
other parts greatly modified; or look at that of the niata ox. Or again, in
the several breeds of the rabbit, observe the elongated skull, with the
differently shaped occipital foramen, atlas, and other cervical vertebræ.
The whole shape of the brain, together with the skull, has been modified in
Polish fowls; in other breeds of the fowl the number of the vertebræ and
the forms of the cervical vertebræ have been changed. In certain pigeons
the shape of the lower jaw, the relative length of the tongue, the size of
the nostrils and eyelids, the number and shape of the ribs, the form and
size of the oesophagus, have all varied. In certain quadrupeds the length
of the intestines has been much increased or {409} diminished. With plants
we see wonderful differences in the stones of various fruits. In the
Cucurbitaceæ several highly important characters have varied, such as the
sessile position of the stigmas on the ovarium, the position of the carpels
within the ovarium, and its projection out of the receptacle. But it would
be useless to run through the many facts given in the earlier chapters.

It is notorious how greatly the mental disposition, tastes, habits,
consensual movements, loquacity or silence, and the tone of voice have
varied and been inherited with our domesticated animals. The dog offers the
most striking instance of changed mental attributes, and these differences
cannot be accounted for by descent from distinct wild types. New mental
characters have certainly often been acquired, and natural ones lost, under
domestication.

New characters may appear and disappear at any stage of growth, and be
inherited at a corresponding period. We see this in the difference between
the eggs of various breeds of the fowl, and in the down on chickens; and
still more plainly in the differences between the caterpillars and cocoons
of various breeds of the silk-moth. These facts, simple as they appear,
throw light on the characters which distinguish the larval and adult states
of natural species, and on the whole great subject of embryology. New
characters are liable to become attached exclusively to that sex in which
they first appeared, or they may be developed in a much higher degree in
the one than the other sex; or again, after having become attached to one
sex, they may be partially transferred to the opposite sex. These facts,
and more especially the circumstance that new characters seem to be
particularly liable, from some unknown cause, to become attached to the
male sex, have an important bearing on the acquirement by animals in a
state of nature of secondary sexual characters.

It has sometimes been said that our domestic productions do not differ in
constitutional peculiarities, but this cannot be maintained. In our
improved cattle, pigs, &c., the period of maturity, including that of the
second dentition, has been much hastened. The period of gestation varies
much, but has been modified in a fixed manner in only one or two cases. In
{410} our poultry and pigeons the acquirement of down and of the first
plumage by the young, and of the secondary sexual characters by the males,
differ. The number of moults through which the larvæ of silk-moths pass,
varies. The tendency to fatten, to yield much milk, to produce many young
or eggs at a birth or during life, differs in different breeds. We find
different degrees of adaptation to climate, and different tendencies to
certain diseases, to the attacks of parasites, and to the action of certain
vegetable poisons. With plants, adaptation to certain soils, as with some
kinds of plums, the power of resisting frost, the period of flowering and
fruiting, the duration of life, the period of shedding the leaves and of
retaining them throughout the winter, the proportion and nature of certain
chemical compounds in the tissues or seeds, all vary.

There is, however, one important constitutional difference between domestic
races and species; I refer to the sterility which almost invariably
follows, in a greater or less degree, when species are crossed, and to the
perfect fertility of the most distinct domestic races, with the exception
of a very few plants, when similarly crossed. It certainly appears a
remarkable fact that many closely allied species which in appearance differ
extremely little should yield when united only a few, more or less sterile
offspring, or none at all; whilst domestic races which differ conspicuously
from each other, are when united remarkably fertile, and yield perfectly
fertile offspring. But this fact is not in reality so inexplicable as it at
first appears. In the first place, it was clearly shown in the nineteenth
chapter that the sterility of crossed species does not closely depend on
differences in their external structure or general constitution, but
results exclusively from differences in the reproductive system, analogous
with those which cause the lessened fertility of the illegitimate unions
and illegitimate offspring of dimorphic and trimorphic plants. In the
second place, the Pallasian doctrine, that species after having been long
domesticated lose their natural tendency to sterility when crossed, has
been shown to be highly probable; we can scarcely avoid this conclusion
when we reflect on the parentage and present fertility of the several
breeds of the dog, of Indian and European cattle, sheep, and pigs. Hence it
would be unreasonable to expect that races formed under domestication {411}
should acquire sterility when crossed, whilst at the same time we admit
that domestication eliminates the normal sterility of crossed species. Why
with closely allied species their reproductive systems should almost
invariably have been modified in so peculiar a manner as to be mutually
incapable of acting on each other--though in unequal degrees in the two
sexes, as shown by the difference in fertility between reciprocal crosses
in the same species--we do not know, but may with much probability infer
the cause to be as follows. Most natural species have been habituated to
nearly uniform conditions of life for an incomparably longer period of time
than have domestic races; and we positively know that changed conditions
exert an especial and powerful influence on the reproductive system. Hence
this difference in habituation may well account for the different action of
the reproductive organs when domestic races and when species are crossed.
It is a nearly analogous fact, that most domestic races may be suddenly
transported from one climate to another, or be placed under widely
different conditions, and yet retain their fertility unimpaired; whilst a
multitude of species subjected to lesser changes are rendered incapable of
breeding.

With the exception of fertility, domestic varieties resemble species when
crossed in transmitting their characters in the same unequal manner to
their offspring, in being subject to the prepotency of one form over the
other, and in their liability to reversion. By repeated crosses a variety
or a species may be made completely to absorb another. Varieties, as we
shall see when we treat of their antiquity, sometimes inherit their new
characters almost, or even quite, as firmly as species. With both, the
conditions leading to variability and the laws governing its nature appear
to be the same. Domestic varieties can be classed in groups under groups,
like species under genera, and these under families and orders; and the
classification may be either artificial,--that is, founded on any arbitrary
character,--or natural. With varieties a natural classification is
certainly founded, and with species is apparently founded, on community of
descent, together with the amount of modification which the forms have
undergone. The characters by which domestic varieties differ from each
other are more {412} variable than those distinguishing species, though
hardly more so than with certain protean species; but this greater degree
of variability is not surprising, as varieties have generally been exposed
within recent times to fluctuating conditions of life, are much more liable
to have been crossed, and are still in many cases undergoing, or have
recently undergone, modification by man's methodical or unconscious
selection.

Domestic varieties as a general rule certainly differ from each other in
less important parts of their organisation than do species; and when
important differences occur, they are seldom firmly fixed; but this fact is
intelligible if we consider man's method of selection. In the living animal
or plant he cannot observe internal modifications in the more important
organs; nor does he regard them as long as they are compatible with health
and life. What does the breeder care about any slight change in the molar
teeth of his pigs, or for an additional molar tooth in the dog; or for any
change in the intestinal canal or other internal organ? The breeder cares
for the flesh of his cattle being well marbled with fat, and for an
accumulation of fat within the abdomen of his sheep, and this he has
effected. What would the floriculturist care for any change in the
structure of the ovarium or of the ovules? As important internal organs are
certainly liable to numerous slight variations, and as these would probably
be inherited, for many strange monstrosities are transmitted, man could
undoubtedly effect a certain amount of change in these organs. When he has
produced any modification in an important part, it has generally been
unintentionally in correlation with some other conspicuous part, as when he
has given ridges and protuberances to the skulls of fowls, by attending to
the form of the comb, and in the case of the Polish fowl to the plume of
feathers on the head. By attending to the external form of the
pouter-pigeon, he has enormously increased the size of the oesophagus, and
has added to the number of the ribs, and given them greater breadth. With
the carrier-pigeon, by increasing, through steady selection, the wattles on
the upper mandible, he has greatly modified the form of the lower mandible;
and so in many other cases. Natural species, on the other hand, have been
modified exclusively for their own good, to fit them for infinitely {413}
diversified conditions of life, to avoid enemies of all kinds, and to
struggle against a host of competitors. Hence, under such complex
conditions, it would often happen that modifications of the most varied
kinds, in important as well as in unimportant parts, would be advantageous
or even necessary; and they would slowly but surely be acquired through the
survival of the fittest. Various indirect modifications would likewise
arise through the law of correlated variation.

Domestic breeds often have an abnormal or semi-monstrous character, as the
Italian greyhound, bulldog, Blenheim spaniel, and bloodhound amongst
dogs,--some breeds of cattle and pigs, several breeds of the fowl, and the
chief breeds of the pigeon. The differences between such abnormal breeds
occur in parts which in closely-allied natural species differ but slightly
or not at all. This may be accounted for by man's often selecting,
especially at first, conspicuous and semi-monstrous deviations of
structure. We should, however, be cautious in deciding what deviations
ought to be called monstrous: there can hardly be a doubt that, if the
brush of horse-like hair on the breast of the turkey-cock had first
appeared on the domesticated bird, it would have been considered a
monstrosity; the great plume of feathers on the head of the Polish cock has
been thus designated, though plumes are common with many kinds of birds; we
might call the wattle or corrugated skin round the base of the beak of the
English carrier-pigeon a monstrosity, but we do not thus speak of the
globular fleshy excrescence at the base of the beak of the male _Carpophaga
oceanica_.

Some authors have drawn a wide distinction between artificial and natural
breeds; although in extreme cases the distinction is plain, in many other
cases an arbitrary line has to be drawn. The difference depends chiefly on
the kind of selection which has been applied. Artificial breeds are those
which have been intentionally improved by man; they frequently have an
unnatural appearance, and are especially liable to loss of excellence
through reversion and continued variability. The so-called natural breeds,
on the other hand, are those which are now found in semi-civilised
countries, and which formerly inhabited separate districts in nearly all
the European kingdoms. They have been rarely acted on by man's {414}
intentional selection; more frequently, it is probable, by unconscious
selection, and partly by natural selection, for animals kept in
semi-civilised countries have to provide largely for their own wants. Such
natural breeds will also, it may be presumed, have been directly acted on
to some extent by the differences, though slight, in the surrounding
physical conditions.

It is a much more important distinction that some breeds have been from
their first origin modified in so slow and insensible a manner, that if we
could see their early progenitors we should hardly be able to say when or
how the breed first arose; whilst other breeds have originated from a
strongly-marked or semi-monstrous deviation of structure, which, however,
may subsequently have been augmented by selection. From what we know of the
history of the racehorse, greyhound, gamecock, &c., and from their general
appearance, we may feel nearly confident that they were formed by a slow
process of improvement: and with the carrier-pigeon, as well as with some
other pigeons, we know that this has been the case. On the other hand, it
is certain that the ancon and mauchamp breeds of sheep, and almost certain
that the niata cattle, turnspit and pug-dogs, jumper and frizzled fowls,
short-faced tumbler pigeons, hook-billed ducks, &c., and with plants a
multitude of varieties, suddenly appeared in nearly the same state as we
now see them. The frequency of these cases is likely to lead to the false
belief that natural species have often originated in the same abrupt
manner. But we have no evidence of the appearance, or at least of the
continued procreation, under nature, of abrupt modifications of structure;
and various general reasons could be assigned against such a belief: for
instance, without separation a single monstrous variation would almost
certainly be soon obliterated by crossing.

On the other hand, we have abundant evidence of the constant occurrence
under nature of slight individual differences of the most diversified
kinds; and thus we are led to conclude that species have generally
originated by the natural selection, not of abrupt modifications, but of
extremely slight differences. This process may be strictly compared with
the slow and gradual improvement of the racehorse, greyhound, and gamecock.
As every detail of structure in each species is closely adapted to its
general {415} habits of life, it will rarely happen that one part alone
will be modified; but the co-adapted modifications, as formerly shown, need
not be absolutely simultaneous. Many variations, however, are from the
first connected by the law of correlation. Hence it follows that even
closely-allied species rarely or never differ from each other by some one
character alone; and this same remark applies to a certain extent to
domestic races; for these, if they differ much, generally differ in many
respects.

Some naturalists boldly insist[928] that species are absolutely distinct
productions, never passing by intermediate links into each other; whilst
they maintain that domestic varieties can always be connected either with
each other or with their parent-forms. But if we could always find the
links between the several breeds of the dog, horse, cattle, sheep, pigs,
&c., the incessant doubts whether they are descended from one or several
species would not have arisen. The greyhound genus, if such a term may be
used, cannot be closely connected with any other breed, unless, perhaps, we
go back to the ancient Egyptian monuments. Our English bulldog also forms a
very distinct breed. In all these cases crossed breeds must of course be
excluded, for the most distinct natural species can thus be connected. By
what links can the Cochin fowl be closely united with others? By searching
for breeds still preserved in distant lands, and by going back to
historical records, tumbler-pigeons, carriers, and barbs can be closely
connected with the parent rock-pigeon; but we cannot thus connect the
turbit or the pouter. The degree of distinctness between the various
domestic breeds depends on the amount of modification which they have
undergone, and especially on the neglect and final extinction of the
linking, intermediate, and less valued forms.

It has often been argued that no light is thrown, from the admitted changes
of domestic races, on the changes which natural species are believed to
undergo, as the former are said to be mere temporary productions, always
reverting, as soon as they become feral, to their pristine form. This
argument has been well combated by Mr. Wallace;[929] and full details were
given in the thirteenth chapter, showing that the tendency to reversion in
feral {416} animals and plants has been greatly exaggerated, though no
doubt to a certain extent it exists. It would be opposed to all the
principles inculcated in this work, if domestic animals, when exposed to
new conditions and compelled to struggle for their own wants against a host
of foreign competitors, were not in the course of time in some manner
modified. It should also be remembered that many characters lie latent in
all organic beings ready to be evolved under fitting conditions; and in
breeds modified within recent times the tendency to reversion is
particularly strong. But the antiquity of various breeds clearly proves
that they remain nearly constant as long as their conditions of life remain
the same.

It has been boldly maintained by some authors that the amount of variation
to which our domestic productions are liable is strictly limited; but this
is an assertion resting on little evidence. Whether or not the amount in
any particular direction is fixed, the tendency to general variability
seems unlimited. Cattle, sheep, and pigs have been domesticated and have
varied from the remotest period, as shown by the researches of Rütimeyer
and others, yet these animals have, within quite recent times, been
improved in an unparalleled degree; and this implies continued variability
of structure. Wheat, as we know from the remains found in the Swiss
lake-habitations, is one of the most anciently cultivated plants, yet at
the present day new and better varieties occasionally arise. It may be that
an ox will never be produced of larger size or finer proportions than our
present animals, or a race-horse fleeter than Eclipse, or a gooseberry
larger than the London variety; but he would be a bold man who would assert
that the extreme limit in these respects has been finally attained. With
flowers and fruit it has repeatedly been asserted that perfection has been
reached, but the standard has soon been excelled. A breed of pigeons may
never be produced with a beak shorter than that of the present short-faced
tumbler, or with one longer than that of the English carrier, for these
birds have weak constitutions and are bad breeders; but the shortness and
length of the beak are the points which have been steadily improved during
at least the last 150 years; and some of the best judges deny that the goal
has yet been reached. We may, also, reasonably suspect, from what {417} we
see in natural species of the variability of extremely modified parts, that
any structure, after remaining constant during a long series of
generations, would, under new and changed conditions of life, recommence
its course of variability, and might again be acted on by selection.
Nevertheless, as Mr. Wallace[930] has recently remarked with much force and
truth, there must be both with natural and domestic productions a limit to
change in certain directions; for instance, there must be a limit to the
fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight to be carried, and the power of
contraction in the muscular fibres. The English racehorse may have reached
this limit; but it already surpasses in fleetness its own wild progenitor,
and all other equine species.

It is not surprising, seeing the great difference between many domestic
breeds, that some few naturalists have concluded that all are descended
from distinct aboriginal stocks, more especially as the principle of
selection has been ignored, and the high antiquity of man, as a breeder of
animals, has only recently become known. Most naturalists, however, freely
admit that various extremely dissimilar breeds are descended from a single
stock, although they do not know much about the art of breeding, cannot
show the connecting links, nor say where and when the breeds arose. Yet
these same naturalists will declare, with an air of philosophical caution,
that they can never admit that one natural species has given birth to
another until they behold all the transitional steps. But fanciers have
used exactly the same language with respect to domestic breeds; thus an
author of an excellent treatise says he will never allow that carrier and
fantail pigeons are the descendants of the wild rock-pigeon, until the
transitions have "actually been observed, and can be repeated whenever man
chooses to set about the task." No doubt it is difficult to realise that
slight changes added up during long centuries can produce such results; but
he who wishes to understand the origin of domestic breeds or natural
species must overcome this difficulty.

The causes inducing and the laws governing variability have been so lately
discussed, that I need here only enumerate the leading points. As
domesticated organisms are much more {418} liable to slight deviations of
structure and to monstrosities, than species living under their natural
conditions, and as widely-ranging species vary more than those which
inhabit restricted areas, we may infer that variability mainly depends on
changed conditions of life. We must not overlook the effects of the unequal
combination of the characters derived from both parents, nor reversion to
former progenitors. Changed conditions have an especial tendency to render
the reproductive organs more or less impotent, as shown in the chapter
devoted to this subject; and these organs consequently often fail to
transmit faithfully the parental characters. Changed conditions also act
directly and definitely on the organisation, so that all or nearly all the
individuals of the same species thus exposed become modified in the same
manner; but why this or that part is especially affected we can seldom or
never say. In most cases, however, of the direct action of changed
conditions, independently of the indirect variability caused by the
reproductive organs being affected, indefinite modifications are the
result; in nearly the same manner as exposure to cold or the absorption of
the same poison affects different individuals in various ways. We have
reason to suspect that an habitual excess of highly nutritious food, or an
excess relatively to the wear and tear of the organisation from exercise,
is a powerful exciting cause of variability. When we see the symmetrical
and complex outgrowths, caused by a minute atom of the poison of a
gall-insect, we may believe that slight changes in the chemical nature of
the sap or blood would lead to extraordinary modifications of structure.

The increased use of a muscle with its various attached parts, and the
increased activity of a gland or other organ, lead to their increased
development. Disuse has a contrary effect. With domesticated productions
organs sometimes become rudimentary through abortion; but we have no reason
to suppose that this has ever followed from mere disuse. With natural
species, on the contrary, many organs appear to have been rendered
rudimentary through disuse, aided by the principle of the economy of
growth, and by the hypothetical principle discussed in the last chapter,
namely, the final destruction of the germs or gemmules of such useless
parts. This difference may be partly {419} accounted for by disuse having
acted on domestic forms for an insufficient length of time, and partly from
their exemption from any severe struggle for existence, entailing rigid
economy in the development of each part, to which all species under nature
are subjected. Nevertheless the law of compensation or balancement
apparently affects, to a certain extent, our domesticated productions.

We must not exaggerate the importance of the definite action of changed
conditions in modifying all the individuals of the same species in the same
manner, or of use and disuse. As every part of the organisation is highly
variable, and as variations are so easily selected, both consciously and
unconsciously, it is very difficult to distinguish between the effects of
the selection of indefinite variations, and the direct action of the
conditions of life. For instance, it is possible that the feet of our
water-dogs, and of the American dogs which have to travel much over the
snow, may have become partially webbed from the stimulus of widely
extending their toes; but it is far more probable that the webbing, like
the membrane between the toes of certain pigeons, spontaneously appeared
and was afterwards increased by the best swimmers and the best
snow-travellers being preserved during many generations. A fancier who
wished to decrease the size of his bantams or tumbler-pigeons would never
think of starving them, but would select the smallest individuals which
spontaneously appeared. Quadrupeds are sometimes born destitute of hair,
and hairless breeds have been formed, but there is no reason to believe
that this is caused by a hot climate. Within the tropics heat often causes
sheep to lose their fleeces, and on the other hand wet and cold act as a
direct stimulus to the growth of hair; it is, however, possible that these
changes may merely be an exaggeration of the regular yearly change of coat;
and who will pretend to decide how far this yearly change, or the thick fur
of arctic animals, or as I may add their white colour, is due to the direct
action of a severe climate, and how far to the preservation of the best
protected individuals during a long succession of generations?

Of all the laws governing variability, that of correlation is the most
important. In many cases of slight deviations of structure as well as of
grave monstrosities, we cannot even {420} conjecture what is the nature of
the bond of connexion. But between homologous parts--between the fore and
hind limbs--between the hair, hoofs, horns, and teeth--we can see that
parts which are closely similar during their early development, and which
are exposed to similar conditions, would be liable to be modified in the
same manner. Homologous parts, from having the same nature, are apt to
blend together and, when many exist, to vary in number.

Although every variation is either directly or indirectly caused by some
change in the surrounding conditions, we must never forget that the nature
of the organisation which is acted on essentially governs the result.
Distinct organisms, when placed under similar conditions, vary in different
manners, whilst closely-allied organisms under dissimilar conditions often
vary in nearly the same manner. We see this in the same modification
frequently reappearing at long intervals of time in the same variety, and
likewise in the several striking cases given of analogous or parallel
varieties. Although some of these latter cases are simply due to reversion,
others cannot thus be accounted for.

From the indirect action of changed conditions on the organisation, through
the impaired state of the reproductive organs--from the direct action of
such conditions (and this will cause the individuals of the same species
either to vary in the same manner, or differently in accordance with slight
differences in their constitution)--from the effects of the increased or
decreased use of parts,--and from correlation,--the variability of our
domesticated productions is complicated in an extreme degree. The whole
organisation becomes slightly plastic. Although each modification must have
its proper exciting cause, and though each is subjected to law, yet we can
so rarely trace the precise relation between cause and effect, that we are
tempted to speak of variations as if they spontaneously arose. We may even
call them accidental, but this must be only in the sense in which we say
that a fragment of rock dropped from a height owes its shape to accident.

It may be worth while briefly to consider the results of the exposure to
unnatural conditions of a large number of animals of the same species,
allowed to cross freely, with no selection of any {421} kind; and
afterwards to consider the results when selection is brought into play. Let
us suppose that 500 wild rock-pigeons were confined in their native land in
an aviary, and fed in the same manner as pigeons usually are; and that they
were not allowed to increase in number. As pigeons propagate so rapidly, I
suppose that a thousand or fifteen hundred birds would have to be annually
killed by mere chance. After several generations had been thus reared, we
may feel sure that some of the young birds would vary, and the variations
would tend to be inherited; for at the present day slight deviations of
structure often occur, but, as most breeds are already well established,
these modifications are rejected as blemishes. It would be tedious even to
enumerate the multitude of points which still go on varying or have
recently varied. Many variations would occur in correlation, as the length
of the wing and tail feathers--the number of the primary wing-feathers, as
well as the number and breadth of the ribs, in correlation with the size
and form of the body--the number of the scutellæ, with the size of the
feet--the length of the tongue, with the length of the beak--the size of
the nostrils and eyelids and the form of lower jaw in correlation with the
development of wattle--the nakedness of the young with the future colour of
the plumage--the size of the feet and beak, and other such points. Lastly,
as our birds are supposed to be confined in an aviary, they would use their
wings and legs but little, and certain parts of the skeleton, such as the
sternum and scapulæ and the feet, would in consequence become slightly
reduced in size.

As in our assumed case many birds have to be indiscriminately killed every
year, the chances are against any new variety surviving long enough to
breed. And as the variations which arise are of an extremely diversified
nature, the chances are very great against two birds pairing which have
varied in the same manner; nevertheless, a varying bird even when not thus
paired would occasionally transmit its character to its young; and these
would not only be exposed to the same conditions which first caused the
variation in question to appear, but would in addition inherit from their
one modified parent a tendency again to vary in the same manner. So that,
if the conditions decidedly tended to induce some particular variation, all
the birds might {422} in the course of time become similarly modified. But
a far commoner result would be, that one bird would vary in one way and
another bird in another way; one would be born with a little longer beak,
and another with a shorter beak; one would gain some black feathers,
another some white or red feathers. And as these birds would be continually
intercrossing, the final result would be a body of individuals differing
from each other slightly in many ways, yet far more than did the original
rock-pigeons. But there would not be the least tendency to the formation of
distinct breeds.

If two separate lots of pigeons were to be treated in the manner just
described, one in England and the other in a tropical country, the two lots
being supplied with different food, would they, after many generations had
passed, differ? When we reflect on the cases given in the twenty-third
chapter, and on such facts as the difference in former times between the
breeds of cattle, sheep, &c., in almost every district of Europe, we are
strongly inclined to admit that the two lots would be differently modified
through the influence of climate and food. But the evidence on the definite
action of changed conditions is in most cases insufficient; and, with
respect to pigeons, I have had the opportunity of examining a large
collection of domesticated birds, sent to me by Sir W. Elliot from India,
and they varied in a remarkably similar manner with our European birds.

If two distinct breeds were to be confined together in equal numbers, there
is reason to suspect that they would to a certain extent prefer pairing
with their own kind; but they would likewise intercross. From the greater
vigour and fertility of the crossed offspring, the whole body would by this
means become interblended sooner than would otherwise have occurred. From
certain breeds being prepotent over others, it does not follow that the
interblended progeny would be strictly intermediate in character. I have,
also, proved that the act of crossing in itself gives a strong tendency to
reversion, so that the crossed offspring would tend to revert to the state
of the aboriginal rock-pigeon. In the course of time they would probably be
not much more heterogeneous in character than in our first case, when birds
of the same breed were confined together. {423}

I have just said that the crossed offspring would gain in vigour and
fertility. From the facts given in the seventeenth chapter there can be no
doubt of this; and there can be little doubt, though the evidence on this
head is not so easily acquired, that long-continued close interbreeding
leads to evil results. With hermaphrodites of all kinds, if the sexual
elements of the same individual habitually acted on each other, the closest
possible interbreeding would be perpetual. Therefore we should bear in mind
that with all hermaphrodite animals, as far as I can learn, their structure
permits and frequently necessitates a cross with a distinct individual.
With hermaphrodite plants we incessantly meet with elaborate and perfect
contrivances for this same end. It is no exaggeration to assert that, if
the use of the talons and tusks of a carnivorous animal, or the use of the
viscid threads of a spider's web, or of the plumes and hooks on a seed may
be safely inferred from their structure, we may with equal safety infer
that many flowers are constructed for the express purpose of ensuring a
cross with a distinct plant. From these various considerations, the
conclusion arrived at in the chapter just referred to--namely, that great
good of some kind is derived from the sexual concourse of distinct
individuals--must be admitted.

To return to our illustration: we have hitherto assumed that the birds were
kept down to the same number by indiscriminate slaughter; but if the least
choice be permitted in their preservation and slaughter, the whole result
will be changed. Should the owner observe any slight variation in one of
his birds, and wish to obtain a breed thus characterised, he would succeed
in a surprisingly short time by carefully selecting and pairing the young.
As any part which has once varied generally goes on varying in the same
direction, it is easy, by continually preserving the most strongly marked
individuals, to increase the amount of difference up to a high,
predetermined standard of excellence. This is methodical selection.

If the owner of the aviary, without any thought of making a new breed,
simply admired, for instance, short-beaked more than long-beaked birds, he
would, when he had to reduce the number, generally kill the latter; and
there can be no doubt that he would thus in the course of time sensibly
modify his {424} stock. It is improbable, if two men were to keep pigeons
and act in this manner, that they would prefer exactly the same characters;
they would, as we know, often prefer directly opposite characters, and the
two lots would ultimately come to differ. This has actually occurred with
strains or families of cattle, sheep, and pigeons, which have been long
kept and carefully attended to by different breeders without any wish on
their part to form new and distinct sub-breeds. This unconscious kind of
selection will more especially come into action with animals which are
highly serviceable to man; for every one tries to get the best dog, horse,
cow, or sheep, and these animals will transmit more or less surely their
good qualities to their offspring. Hardly any one is so careless as to
breed from his worst animals. Even savages, when compelled from extreme
want to kill some of their animals, would destroy the worst and preserve
the best. With animals kept for use and not for mere amusement, different
fashions prevail in different districts, leading to the preservation, and
consequently to the transmission, of all sorts of trifling peculiarities of
character. The same process will have been pursued with our fruit-trees and
vegetables, for the best will always have been the most largely cultivated,
and will occasionally have yielded seedlings better than their parents.

The different strains, just alluded to, which have been raised by different
breeders without any wish for such a result, and the unintentional
modification of foreign breeds in their new homes, both afford excellent
evidence of the power of unconscious selection. This form of selection has
probably led to far more important results than methodical selection, and
is likewise more important under a theoretical point of view from closely
resembling natural selection. For during this process the best or most
valued individuals are not separated and prevented crossing with others of
the same breed, but are simply preferred and preserved; but this inevitably
leads during a long succession of generations to their increase in number
and to their gradual improvement; so that finally they prevail to the
exclusion of the old parent-form.

With our domesticated animals natural selection checks the production of
races with any injurious deviation of {425} structure. In the case of
animals kept by savages and semi-civilised people, which have to provide
largely for their own wants under different circumstances, natural
selection will probably play a more important part. Hence such animals
often closely resemble natural species.

As there is no limit to man's desire to possess animals and plants more and
more useful in any respect, and as the fancier always wishes, from fashion
running into extremes, to produce each character more and more strongly
pronounced, there is a constant tendency in every breed, through the
prolonged action of methodical and unconscious selection, to become more
and more different from its parent-stock; and when several breeds have been
produced and are valued for different qualities, to differ more and more
from each other. This leads to Divergence of Character. As improved
sub-varieties and races are slowly formed, the older and less improved
breeds are neglected and decrease in number. When few individuals of any
breed exist within the same locality, close interbreeding, by lessening
their vigour and fertility, aids in their final extinction. Thus the
intermediate links are lost, and breeds which have already diverged gain
Distinctness of Character.

In the chapters on the Pigeon, it was proved by historical details and by
the existence of connecting sub-varieties in distant lands that several
breeds have steadily diverged in character, and that many old and
intermediate sub-breeds have become extinct. Other cases could be adduced
of the extinction of domestic breeds, as of the Irish wolf-dog, the old
English hound, and of two breeds in France, one of which was formerly
highly valued.[931] Mr. Pickering remarks[932] that "the sheep figured on
the most ancient Egyptian monuments is unknown at the present day; and at
least one variety of the bullock, formerly known in Egypt, has in like
manner become extinct." So it has been with some animals, and with several
plants cultivated by the ancient inhabitants of Europe during the neolithic
period. In Peru, Von Tschudi[933] found in certain tombs, apparently prior
to the dynasty of the Incas, two kinds of maize not now known in the
country. With our flowers and culinary vegetables, {426} the production of
new varieties and their extinction has incessantly recurred. At the present
time improved breeds sometimes displace at an extraordinarily rapid rate
older breeds; as has recently occurred throughout England with pigs. The
Long-horn cattle in their native home were "suddenly swept away as if by
some murderous pestilence," by the introduction of Short-horns.[934]

What grand results have followed from the long-continued action of
methodical and unconscious selection, checked and regulated to a certain
extent by natural selection, is seen on every side of us. Compare the many
animals and plants which are displayed at our exhibitions with their
parent-forms when these are known, or consult old historical records with
respect to their former state. Almost all our domesticated animals have
given rise to numerous and distinct races, excepting those which cannot be
easily subjected to selection--such as cats, the cochineal insect, and the
hive-bee,--and excepting those animals which are not much valued. In
accordance with what we know of the process of selection, the formation of
our many races has been slow and gradual. The man who first observed and
preserved a pigeon with its oesophagus a little enlarged, its beak a little
longer, or its tail a little more expanded than usual, never dreamed that
he had made the first step in the creation of the pouter, carrier, and
fantail-pigeon. Man can create not only anomalous breeds, but others with
their whole structure admirably co-ordinated for certain purposes, such as
the race-horse and dray-horse, or the greyhound. It is by no means
necessary that each small change of structure throughout the body, leading
towards excellence, should simultaneously arise and be selected. Although
man seldom attends to differences in organs which are important under a
physiological point of view, yet he has so profoundly modified some breeds,
that assuredly, if found wild, they would be ranked under distinct genera.

The best proof of what selection has effected is perhaps afforded by the
fact that whatever part or quality in any animal, and more especially in
any plant, is most valued by man, that part or quality differs most in the
several races. This result is well seen by comparing the amount of
difference {427} between the fruits produced by the varieties of the same
fruit-tree, between the flowers of the varieties in our flower-garden,
between the seeds, roots, or leaves of our culinary and agricultural
plants, in comparison with the other and not valued parts of the same
plants. Striking evidence of a different kind is afforded by the fact
ascertained by Oswald Heer,[935] namely, that the seeds of a large number
of plants,--wheat, barley, oats, peas, beans, lentils, poppies,--cultivated
for their seed by the ancient Lake-inhabitants of Switzerland, were all
smaller than the seeds of our existing varieties. Rütimeyer has shown that
the sheep and cattle which were kept by the earlier Lake-inhabitants were
likewise smaller than our present breeds. In the middens of Denmark, the
earliest dog of which the remains have been found was the weakest; this was
succeeded during the Bronze age by a stronger kind, and this again during
the Iron age by one still stronger. The sheep of Denmark during the Bronze
period had extraordinarily slender limbs, and the horse was smaller than
our present animal.[936] No doubt in these cases the new and larger breeds
were generally introduced from foreign lands by the immigration of new
hordes of men. But it is not probable that each larger breed, which in the
course of time supplanted a previous and smaller breed, was the descendant
of a distinct and larger species; it is far more probable that the domestic
races of our various animals were gradually improved in different parts of
the great Europæo-Asiatic continent, and thence spread to other countries.
This fact of the gradual increase in size of our domestic animals is all
the more striking as certain wild or half-wild animals, such as red-deer,
aurochs, park-cattle, and boars,[937] have within nearly the same period
decreased in size.