lead to an unusual secretion of an essential oil in the seeds; rendering
them unpalatable to creatures which would otherwise feed on them: so giving
an advantage to the variety in its rate of multiplication. This incidental
peculiarity, proving a preservative, will, as before, be increased by
natural selection until it constitutes another divergence. Now in such
cases, we see that plants may become better adapted, or re-adapted, to the
aggregate of surrounding agencies, not through any _direct_ action of such
agencies on them, but through their _indirect_ action--through the
destruction by them of the individuals least congruous with them, and the
survival of those most congruous with them. All these slight variations of
function and structure, arising among the members of a species, serve as so
many experiments; the great majority of which fail, but a few of which
succeed. Just as each plant bears a multitude of seeds, out of which some
two or three happen to fulfil all the conditions required for reaching
maturity and continuing the race; so each species is ever producing
numerous slightly-modified forms, deviating in all directions from the
average, out of which most fit the surrounding conditions no better than
their parents, or not so well, but some few of which fit the conditions
better; and, doing so, are enabled the better to preserve themselves, and
to produce offspring similarly capable of preserving themselves.  Among
animals the like process results in the like development of various
structures which cannot have been affected by the performance of
functions--their functions being purely passive. The thick shell of a
mollusk cannot have arisen from direct reactions of the organism against
the external actions to which it is exposed; but it is quite explicable as
an effect of the survival, generation after generation, of individuals
whose thicker coverings protected them against enemies. Similarly with such
dermal structure as that of the tortoise. Though we have evidence that the
skin, where it is continually exposed to pressure and friction, may
thicken, and so re-establish the equilibrium by opposing a greater inner
force to a greater outer force; yet we have no evidence that a coat of
armour like that of the tortoise can be so produced. Nor, indeed, are the
conditions under which alone its production in such a manner could be
accounted for, fulfilled; since the surface of the tortoise is not exposed
to greater pressure and friction than the surfaces of other creatures. This
massive carapace, and the strangely-adapted osseous frame-work which
supports it, are inexplicable as results of evolution, unless through the
process of natural selection. So, too, is it with the formation of
odoriferous glands in some mammals, or the growth of such excrescences as
those of the camel. Thus, in short, is it with all those organs of animals
which do not play active parts.

Besides giving us explanations of structural characters that are otherwise
unaccountable, Mr. Darwin shows how natural selection explains peculiar
relations between individuals in certain species. Such facts as the
dimorphism of the primrose and other flowers, he proves to be in harmony
with his hypothesis, though stumbling-blocks to all other hypotheses. The
various differences which accompany difference of sex, sometimes slight,
sometimes very great, are similarly accounted for. As before suggested
(§ 79), natural selection appears capable of producing and maintaining the
right proportion of the sexes in each species; and it requires but to
contemplate the bearings of the argument, to see that the formation of
different sexes may itself have been determined in the same way.

To convey here an adequate idea of Mr. Darwin's doctrine, throughout the
immense range of its applications, is of course impossible. The few
illustrations just given, are intended simply to remind the reader what Mr.
Darwin's hypothesis is, and what are the else insoluble problems which it
solves for us.

§ 166. But now, though it seems to me that we are thus supplied with a key
to phenomena which are multitudinous and varied beyond all conception; it
also seems to me that there is a moiety of the phenomena which this key
will not unlock. Mr. Darwin himself recognizes use and disuse of parts, as
causes of modifications in organisms; and does this, indeed, to a greater
extent than do some who accept his general conclusion. But I conceive that
he does not recognize them to a sufficient extent. While he shows that the
inheritance of changes of structure caused by changes of function, is
utterly insufficient to explain a great mass--probably the greater mass--of
morphological phenomena; I think he leaves unconsidered a mass of
morphological phenomena which are explicable as results of
functionally-produced modifications, and are not explicable as results of
natural selection.

By induction, as well as by inference from the hypothesis of natural
selection, we know that there exists a balance among the powers of organs
which habitually act together--such proportions among them that no one has
any considerable excess of efficiency. We see, for example, that throughout
the vascular system there is maintained an equilibrium of the component
parts: in some cases, under continued excess of exertion, the heart gives
way, and we have enlargement; in other cases the large arteries give way,
and we have aneurisms; in other cases the minute blood-vessels give
way--now bursting, now becoming chronically congested. That is to say, in
the average constitution, no superfluous strength is possessed by any of
the appliances for circulating the blood. Take, again, a set of motor
organs. Great strain here causes the fibres of a muscle to tear. There the
muscle does not yield but the tendon snaps. Elsewhere neither muscle nor
tendon is damaged, but the bone breaks. Joining with these instances the
general fact that, under the same adverse conditions, different individuals
show their slight differences of constitution by going wrong some in one
way and some in another; and that even in the same individual, similar
adverse conditions will now affect one viscus and now another; it becomes
manifest that though there cannot be maintained an accurate balance among
the powers of the organs composing an organism, yet their excesses and
deficiencies of power are extremely slight. That they must be extremely
slight, is, as before said, a deduction from the hypothesis of natural
selection. Mr. Darwin himself argues "that natural selection is continually
trying to economise in every part of the organization. If under changed
conditions of life a structure before useful becomes less useful, any
diminution, however slight, in its development, will be seized on by
natural selection, for it will profit the individual not to have its
nutriment wasted in building up an useless structure." In other words, if
any muscle has more fibres than are required, or if a bone is stronger than
needful, no advantage results but rather a disadvantage--a disadvantage
which will decrease the chances of survival. Hence it follows that among
any organs which habitually act in concert, an increase of one can be of no
service unless there is a concomitant increase of the rest. The
co-operative parts must vary together; otherwise variation will be
detrimental. A stronger muscle must have a stronger bone to resist its
contractions; must have stronger correlated muscles and ligaments to secure
the neighbouring articulations; must have larger blood-vessels to bring it
supplies; must have a more massive nerve to transmit stimulus, and some
extra development of a nervous centre to supply the extra stimulus. The
question arises, then,--do variations of the appropriate kinds occur
simultaneously in all these co-operative parts? Have we any reason to think
that the parts spontaneously increase or decrease together? The assumption
that they do seems to me untenable; and its untenability will, I think,
become conspicuous if we take a case, and observe how extremely numerous
and involved are the variations which must be supposed to occur together.
In illustration of another point, we have already considered the
modifications required to accompany increased weight of the head (§ 155).
Instead of the bison, the moose deer, or the extinct Irish elk, will here
best serve our purpose. In this last species the male has
enormously-developed horns, used for purposes of offence and defence. These
horns, weighing upwards of a hundred-weight, are carried at great
mechanical disadvantage: supported as they are, along with the massive
skull which bears them, at the extremity of the outstretched neck. Further,
that these heavy horns may be of use in fighting, the supporting bones and
muscles must be strong enough, not simply to carry them, but to put them in
motion with the rapidity needed for giving blows. Let us, then, ask how, by
natural selection, this complex apparatus of bones and muscles can have
been developed, _pari passu_ with the horns? If we suppose the horns to
have been originally of like size with those borne by other kinds of deer;
and if we suppose that in some individual they became larger by spontaneous
variation; what would be the concomitant changes required to render their
greater size useful? Other things equal, the blow given by a larger horn
would be a blow given by a heavier mass moving at a smaller velocity: the
momentum would be the same as before; and the area of contact with the body
struck being somewhat increased, while the velocity was decreased, the
injury done would be less. That horns may become better weapons, the whole
apparatus concerned in moving them must be so strengthened as to impress
more force on them, and to bear the more violent reactions of the blows
given. The bones of the skull on which the horns are seated must be
thickened; otherwise they will break. The vertebræ of the neck must be
further developed; and unless the ligaments which hold together these
vertebræ, and the muscles which move them, are also enlarged, nothing will
be gained. Again the upper dorsal vertebræ and their spines must be
strengthened, that they may withstand the stronger contractions of the
neck-muscles; and like changes must be made on the scapular arch. Still
more must there be required a simultaneous development of the bones and
muscles of the fore-legs; since these extra growths in the horns, in the
skull, in the neck, in the shoulders, add to the burden they have to bear;
and without they are strengthened the creature will not only suffer from
loss of speed but will fail in fight. Hence, to make larger horns of use,
additional sizes must be acquired by numerous bones, muscles, and
ligaments, as well as by the blood-vessels and nerves on which their
actions depend. On calling to mind how the spraining of a single small
muscle in the foot incapacitates for walking, or how permanent weakness in
a knee-ligament will diminish the power of the leg, it will be seen that
unless all these many changes are simultaneously made, they may as well be
none of them made--or rather, they would better be none of them made; since
the enlargements of some parts, by putting greater strains on connected
parts, would render them relatively weaker if they remained unenlarged. Can
we with any propriety assume that these many enlargements duly proportioned
will be simultaneously effected by spontaneous variations? I think not. It
would be a strong supposition that the vertebræ and muscles of the neck
suddenly became bigger at the same time as the horns. It would be a still
stronger supposition that the upper dorsal vertebræ not only at the same
time became more massive, but appropriately altered their proportions, by
the development of their immense neural spines. And it would be an
assumption still more straining our powers of belief, that along with
heavier horns there should spontaneously take place the required
strengthenings in the bones, muscles, arteries, and nerves of the scapular
and the fore-legs.

Besides the multiplicity of directly-coöperative organs, the multiplicity
of organs which do not coöperate, save in the degree implied by their
combination in the same organism, seems to me a further hindrance to the
development of special structures by natural selection alone. Where the
life is simple, or where circumstances render some one function supremely
important, survival of the fittest may readily bring about the appropriate
structural change, without aid from the transmission of functionally-caused
modifications. But in proportion as the life grows complex--in proportion
as a healthy existence cannot be secured by a large endowment of some one
power, but demands many powers; in the same proportion do there arise
obstacles to the increase of any particular power by "the preservation of
favoured races in the struggle for life." As fast as the faculties are
multiplied, so fast does it become possible for the several members of a
species to have various kinds of superiorities over one another. While one
saves its life by higher speed, another does the like by clearer vision,
another by keener scent, another by quicker hearing, another by greater
strength, another by unusual power of enduring cold or hunger, another by
special sagacity, another by special timidity, another by special courage;
and others by other bodily and mental attributes. Conditions being alike,
each of these life-saving attributes is likely to be transmitted to
posterity. But we may not assume that it will be increased in subsequent
generations by natural selection. Increase of it can result only if
individuals possessing average endowments of it are more frequently killed
off than individuals highly endowed with it; and this can happen only when
the attribute is one of greater importance, for the time being, than most
of the other attributes. If those members of the species which have but
ordinary shares of it, nevertheless survive by virtue of other
superiorities which they severally possess; then it is not easy to see how
this particular attribute can be developed by natural selection in
subsequent generations. The probability seems rather to be that, by
gamogenesis, this extra endowment will, on the average, be diminished in
posterity--just serving in the long run to make up for the deficient
endowments of those whose special powers lie in other directions; and so to
keep up the normal structure of the species. As fast as the number of
bodily and mental faculties increases, and as fast as maintenance of life
comes to depend less on the amount of any one and more on the combined
actions of all; so fast does the production of specialities of character by
natural selection alone, become difficult. Particularly does this seem to
be so with a species so multitudinous in its powers as mankind; and above
all does it seem to be so with such of the human powers as have but minor
shares in aiding the struggle for life--the æsthetic faculties, for
example.

It by no means follows, however, that in cases of this kind, and cases of
the preceding kind, natural selection plays no part. Wherever it is not the
chief agent in working organic changes, it is still, very generally, a
secondary agent. The survival of the fittest must nearly always further the
production of modifications which produce fitness, whether they be
incidental modifications, or modifications caused by direct adaptation.
Evidently, those individuals whose constitutions have facilitated the
production in them of any structural change consequent on any functional
change demanded by some new external condition, will be the individuals
most likely to live and to leave descendants. There must be a natural
selection of functionally-acquired peculiarities, as well as of
spontaneously-acquired peculiarities; and hence such structural changes in
a species as result from changes of habit necessitated by changed
circumstances, natural selection will render more rapid than they would
otherwise be.

There are, however, some modifications in the sizes and forms of parts,
which cannot have been aided by natural selection; but which must have
resulted wholly from the inheritance of functionally-caused alterations.
The dwindling of organs of which the undue sizes entail no appreciable
evils, furnishes the best evidence of this. Take, for an example, that
diminution of the jaws and teeth which characterizes the civilized races,
as contrasted with the savage races.[53] How can the civilized races have
been benefited in the struggle for life, by the slight decrease in these
comparatively-small bones? No functional superiority possessed by a small
jaw over a large jaw in civilized life, can be named as having caused the
more frequent survival of small-jawed individuals. The only advantage
accompanying smallness of jaw, is the advantage of economized nutrition;
and this cannot be great enough to further the preservation of those
distinguished by it. The decrease of weight in the jaw and co-operative
parts, which has arisen in the course of thousands of years, does not
amount to more than a few ounces. This decrease has to be divided among the
many generations which have lived and died in the interval. Let us admit
that the weight of these parts diminished to the extent of an ounce in a
single generation (which is a large admission); it still cannot be
contended that the having to carry an ounce less in weight, and to keep in
repair an ounce less of tissue, could sensibly affect any man's fate. And
if it never did this--nay, if it did not cause a _frequent_ survival of
small-jawed individuals where large-jawed individuals died; natural
selection could neither cause nor aid diminution of the jaw and its
appendages. Here, therefore, the decreased action which has accompanied the
growth of civilized habits (the use of tools and the disuse of coarse
food), must have been the sole cause at work. Through direct equilibration,
diminished external stress on these parts has resulted in diminution of the
internal forces by which this stress is met. From generation to generation,
this lessening of the parts consequent on functional decline has been
inherited. And since the survival of individuals must always have been
determined by more important structural traits, this trait can have neither
been facilitated nor retarded by natural selection.

§ 167. Returning from these extensive classes of facts for which Mr.
Darwin's hypothesis does not account, to the still more extensive classes
of facts for which it does account, and which are unaccountable on any
other hypothesis; let us consider in what way this hypothesis is
expressible in terms of the general doctrine of evolution. Already it has
been pointed out that the evolving of modified types by "natural selection
or the preservation of favoured races in the struggle for life," must be a
process of equilibration; since it results in the production of organisms
which are in equilibrium with their environments. At the outset of this
chapter, something was done towards showing how this continual survival of
the fittest may be understood as the progressive establishment of a balance
between inner and outer forces. Here, however, we must consider the matter
more closely.

On previous occasions we have contemplated the assemblage of individuals
composing a species, as an aggregate in a state of moving equilibrium. We
have seen that its powers of multiplication give it an expansive energy
which is antagonized by other energies; and that through the rhythmical
variations in these two sets of energies there is maintained an oscillating
limit to its habitat, and an oscillating limit to its numbers. On another
occasion (§ 96) it was shown that the aggregate of individuals constituting
a species, has a kind of general life which, "like the life of an
individual, is maintained by the unequal and ever-varying actions of
incident forces on its different parts." We saw that "just as, in each
organism, incident forces constantly produce divergences from the mean
state in various directions, which are constantly balanced by opposite
divergences indirectly produced by other incident forces; and just as the
combination of rhythmical functions thus maintained, constitutes the life
of the organism; so, in a species there is, through gamogenesis, a
perpetual neutralization of those contrary deviations from the mean state,
which are caused in its different parts by different sets of incident
forces; and it is similarly by the rhythmical production and compensation
of these contrary deviations that the species continues to live." Hence, to
understand how a species is affected by causes which destroy some of its
units and favour the multiplication of others, we must consider it as a
whole whose parts are held together by complex forces that are ever
re-balancing themselves--a whole whose moving equilibrium is continually
disturbed and continually rectified. Thus much premised, let us next call
to mind how moving equilibria in general are changed. In the first place, a
new incident force falling on any part of an aggregate with balanced
motions, produces a new motion in the direction of least resistance. In the
second place, the new incident force is gradually used up in overcoming the
opposing forces, and when it is all expended the opposing forces produce a
recoil--a reverse deviation which counter-balances the original deviation.
Consequently, to consider whether the moving equilibrium of a species is
modified in the same way as moving equilibria in general, is to consider
whether, when exposed to a new force, a species yields in the direction of
least resistance; and whether, by its thus yielding, there is generated in
the species a compensating change in the opposite direction. We shall find
that it does both these things.

For what, expressed in mechanical terms, is the effect wrought on a species
by some previously-unknown enemy, that kills such of its members as fail in
defending themselves? The disappearance of those individuals which meet the
destroying forces by the smallest preserving forces, is tantamount to the
yielding of the species as a whole at the places where the resistances are
the least. Or if by some general influence, such as alteration of climate,
the members of a species are subject to increase of external actions which
are ever tending to overthrow their equilibria, and which they are ever
counter-balancing by certain physiological actions, which are the first to
die? Those least able to generate the internal energies which antagonize
these external energies. If the change be an increase of the winter's cold,
then such members of the species as have unusual powers of getting food or
of digesting food, or such as are by their constitutional aptitude for
making fat, furnished with reserve stores of force, available in times of
scarcity, or such as have the thickest coats and so lose least heat by
radiation, survive; and their survival implies that in each of them the
moving equilibrium of functions presents such an adjustment of internal
forces, as prevents overthrow by the modified aggregate of external forces.
Conversely, the members which die are, other things equal, those deficient
in the power of meeting the new action by an equivalent counter-action.
Thus, in all cases, a species considered as an aggregate in a state of
moving equilibrium, has its state changed by the yielding of its
fluctuating mass wherever this mass is weakest in relation to the special
forces acting on it. The conclusion is, indeed, a truism. But now what must
follow from the destruction of the least-resisting individuals and survival
of the most-resisting individuals? On the moving equilibrium of the species
as a whole, existing from generation to generation, the effect of this
deviation from the mean state is to produce a compensating deviation. For
if all such as are deficient of power in a certain direction are destroyed,
what must be the effect on posterity? Had they lived and left offspring,
the next generation would have had the same average powers as preceding
generations: there would have been a like proportion of individuals less
endowed with the needful power, and individuals more endowed with it. But
the more-endowed individuals being alone left to continue the race, there
must result a new generation characterized by a larger average endowment of
this power. That is to say, on the moving equilibrium of a species, an
action producing change in a given direction is followed, in the next
generation, by a reaction producing an opposite change. Observe, too, that
these effects correspond in their degrees of violence.  If the alteration
of some external factor is so great that it leaves alive only the few
individuals possessing extreme endowments of the power required to
antagonize it; then, in succeeding generations, there is a rapid
multiplication of individuals similarly possessing extreme endowments of
this power--the force impressed calls out an equivalent conflicting force.
Moreover, the change is temporary where the cause is temporary, and
permanent where the cause is permanent. All that are deficient in the
needful attribute having been killed off, and the survivors having the
needful attribute in a comparatively high degree, there will descend from
them, not only some possessing equal amounts of this attribute with
themselves, but also some possessing less amounts of it. If the destructive
agency has not continued in action, such less-endowed individuals will
multiply; and the species, after sundry oscillations, will return to its
previous mean state. But if this agency be a persistent one, such less
endowed individuals will be continually killed off, and eventually none but
highly-endowed individuals will be produced--a new moving equilibrium,
adapted to the new environing conditions, will result.

It may be objected that this mode of expressing the facts does not include
the cases in which a species becomes modified in relation to surrounding
agencies of a passive kind--cases like that of a plant which acquires
hooked seed-vessels, by which it lays hold of the skins of passing animals,
and makes them the distributors of its seeds--cases in which the outer
agency has no direct tendency at first to affect the species, but in which
the species so alters itself as to take advantage of the outer agency. To
cases of this kind, however, the same mode of interpretation applies on
simply changing the terms. While, in the aggregate of influences amid which
a species exists, there are some which tend to overthrow the moving
equilibria of its members, there are others which facilitate the
maintenance of their moving equilibria, and some which are capable of
giving their moving equilibria increased stability: instance the spread
into their habitat of some new kind of prey, which is abundant at seasons
when other prey is scarce. Now what is the process by which the moving
equilibrium in any species becomes adapted to some additional external
factor furthering its maintenance? Instead of an increased resistance to be
met and counterbalanced, there is here a diminished resistance; and the
diminished resistance is equilibrated in the same way as the increased
resistance. As, in the one case, there is a more frequent survival of
individuals whose peculiarities enable them to resist the new adverse
factor; so, in the other case, there is a more frequent survival of
individuals whose peculiarities enable them to take advantage of the new
favourable factor. In each member of the species, the balance of functions
and correlated arrangement of structures, differ slightly from those
existing in other members. To say that among all its members, one is better
fitted than the rest to benefit by some before-unused agency in the
environment, is to say that its moving equilibrium is, in so far, more
stably adjusted to the sum of surrounding influences. And if, consequently,
this individual maintains its moving equilibrium when others fail, and has
offspring which do the like--that is, if individuals thus characterized
multiply and supplant the rest; there is, as before, a process which
effects equilibration between the organism and its environment, not
immediately but mediately, through the continuous intercourse between the
species as a whole and the environment.

§ 168. Thus we see that indirect equilibration does whatever direct
equilibration cannot do. All these processes by which organisms are
re-fitted to their ever-changing environments, must be equilibrations of
one kind or other. As authority for this conclusion, we have not simply the
universal truth that change of every order is towards equilibrium; but we
have also the truth that life itself is a moving equilibrium between inner
and outer actions--a continuous adjustment of internal relations to
external relations; or the maintenance of a balance between the forces to
which an organism is subject and the forces which it evolves. Hence all
changes which enable a species to live under altered conditions, are
changes towards equilibrium with the altered conditions; and therefore
those which do not come within the class of direct equilibrations, must
come within the class of indirect equilibrations.

And now we reach an interpretation of Natural Selection regarded as a part
of Evolution at large. As understood in _First Principles_, Evolution is a
continuous redistribution of matter and motion; and a process of evolution
which is not expressible in terms of matter and motion has not been reduced
to its ultimate form. The conception of Natural Selection is manifestly one
not known to physical science: its terms are not of a kind physical science
can take cognisance of. But here we have found in what manner it may be
brought within the realm of physical science. Rejecting metaphor we see
that the process called Natural Selection is literally a survival of the
fittest; and the outcome of the above argument is that survival of the
fittest is a maintenance of the moving equilibrium of the functions in
presence of outer actions: implying the possession of an equilibrium which
is relatively stable in contrast with the unstable equilibria of those
which do not survive.

CHAPTER XIII.

THE CO-OPERATION OF THE FACTORS.

§ 169. Thus the phenomena of Organic Evolution may be interpreted in the
same way as the phenomena of all other Evolution. Fully to see this, it
will be needful for us to contemplate in their _ensemble_, the several
processes separately described in the four preceding chapters.

If the forces acting on any aggregate remain the same, the changes produced
by them will presently reach a limit, at which the outer forces are
balanced by the inner forces; and thereafter no further metamorphosis will
take place. Hence, that there may be continuous changes of structure in
organisms, there must be continuous changes in the incident forces. This
condition to the evolution of animal and vegetal forms, we find to be fully
satisfied. The astronomic, geologic, and meteorologic changes that have
been slowly but incessantly going on, and have been increasing in the
complexity of their combinations, have been perpetually altering the
circumstances of organisms; and organisms, becoming more numerous in their
kinds and higher in their kinds, have been perpetually altering one
another's circumstances. Thus, for those progressive modifications upon
modifications which organic evolution implies, we find a sufficient cause.
The increasing inner changes for which we thus find a cause in the
perpetual outer changes, conform, so far as we can trace them, to the
universal law of the instability of the homogeneous. In organisms, as in
all other things, the exposure of different parts to different kinds and
amounts of incident forces, has necessitated their differentiation; and,
for the like reason, aggregates of individuals have been lapsing into
varieties, and species, and genera, and orders. Further, in each type of
organism, as in the aggregate of types, the multiplication of effects has
continually aided this transition from a more homogeneous to a more
heterogeneous state. And yet again, that increasing segregation and
concomitant increasing definiteness, associated with the growing
heterogeneity of organisms, has been aided by the continual destruction of
those which expose themselves to aggregates of external actions markedly
incongruous with the aggregates of their internal actions, and the survival
of those subject only to comparatively small incongruities. Finally, we
have found that each change of structure, superposed on preceding changes,
has been a re-equilibration necessitated by the disturbance of a preceding
equilibrium. The maintenance of life being the maintenance of a balanced
combination of functions, it follows that individuals and species that have
continued to live, are individuals and species in which the balance of
functions has not been overthrown. Hence survival through successive
changes of conditions, implies successive adjustments of the balance to the
new conditions.

The actions that are here specified in succession, are in reality
simultaneous; and they must be so conceived before organic evolution can be
rightly understood. Some aid towards so conceiving them will be given by
the annexed table, representing the co-operation of the factors.

§ 170. Respecting this co-operation, it remains only to point out the
respective shares of the factors in producing the total result; and the way
in which the proportions of their respective shares vary as evolution
progresses.

                 Astronomic   }
                 changes      }
                              } alter the    }
                 Geologic     } incidence    }
                 changes      } of inorganic }
                              } forces.      }
                 Meteorologic }              }
                 changes      }              }
                                             }
                                             }
                                             } on each species: affecting
                                             }                    |
                                             }                    |
                                             }                    |
                                             }                    |
                                             }                    |
  Enemies      }                             }                    |
  Competitors  }                             }                    |
               }   varying in }              }                    |
  Co-operators }   number     }              }                    |
  Prey         }              } alter the    }                    |
                              } incidence    }                    |
  Enemies      }              } of organic   }                    |
  Competitors  }              } forces.      }                    |
               }   varying in }                                   |
  Co-operators }   kind       }                                   |
  Prey         }                                                  |
  -----------------------------------------------------------------
  |                                    { which, partially in the first
  |                                    { generation, and completely in
  |                                    { the course of generations, are
  |                                    { directly equilibrated with the
  |                                    { changed agencies.
  |                    { immediately   {
  |                    { through their { which have their direct
  |                    { functions;    { equilibration with the changed
  |                    {               { agencies, aided by indirect
  |                    {               { equilibration, through the more
  |                    {               { frequent survival of those in
  |                    {               { which the direct equilibration
  |                    {               { is most rapid.
  |                    {
  | { its individuals, {               { positively--by aiding the
  | {                  {               { multiplication of those whose
  | {                  {               { moving equilibria happen to be
  | {                  {               { most congruous with the
  | {                  { mediately     { changed agencies: thus, in the
  | {                  { through the   { course of generations, indirectly
  | {                  { aggregate of  { equilibrating certain individuals
  | {                  { individuals;  { with them.
  --{                                  {
    {                                  { negatively--by killing those
    {                                  { whose moving equilibria are
    {                                  { most incongruous with the
    {                                  { changed agencies: thus, in
    {                                  { the course of generations,
    {                                  { indirectly equilibrating each
    {                                  { of its surviving individuals
    {                                  { with them.
    {
    {                 { by acting on it in some parts of the habitat
    {                 { more than in others; and thus differentiating
    { its aggregate   { the species into local varieties.
    { of individuals, {
                      {                       { and thus causing
                      {                       { differentiations of
                      {                       { the species into
                      { by acting differently { varieties, irrespective
                      { on slightly-unlike    { of locality.
                      { individuals in the    {
                      { same locality;        { and thus causing
                                              { modification of the
                                              { species as a whole,
                                              { by abstracting a
                                              { certain class of
                                              { its units.

At first, changes in the amounts and combinations of inorganic forces,
astronomic, geologic, and meteorologic, were the only causes of the
successive modifications; and these changes have continued to be causes.
But as, through the diffusion of organisms and consequent differential
actions of inorganic forces, there arose unlikenesses among them, producing
varieties, species, genera, orders, classes, the actions of organisms on
one another became new sources of organic modifications. And as fast as
types have multiplied and become more complex, so fast have the mutual
actions of organisms come to be more influential factors in their
respective evolutions: eventually becoming the chief factors.

Passing from the external causes of change to the internal processes of
change entailed by them, we see that these, too, have varied in their
proportions: that which was originally the most important and almost the
sole process, becoming gradually less important, if not at last the least
important. Always there must have been, and always there must continue to
be, a survival of the fittest; natural selection must have been in
operation at the outset, and can never cease to operate. While yet
organisms had small abilities to coordinate their actions, and adjust them
to environing actions, natural selection worked almost alone in moulding
and remoulding organisms into fitness for their changing environments; and
natural selection has remained almost the sole agency by which plants and
inferior orders of animals have been modified and developed. The
equilibration of organisms that are almost passive, is necessarily effected
indirectly, by the action of incident forces on the species as a whole. But
along with the evolution of organisms having some activity, there grows up
a kind of equilibration which is in part direct. In proportion as the
activity increases direct equilibration plays a more important part. Until,
when the nervo-muscular apparatus becomes greatly developed, and the power
of varying the actions to fit the varying requirements becomes
considerable, the share taken by direct equilibration rises into
co-ordinate importance or greater importance. As fast as essential
faculties multiply, and as fast as the number of organs which co-operate in
any given function increases, indirect equilibration through natural
selection becomes less and less capable of producing specific adaptations;
and remains capable only of maintaining the general fitness of constitution
to conditions. The production of adaptations by direct equilibration then
takes the first place: indirect equilibration serving to facilitate it.
Until at length, among the civilized human races, the equilibration becomes
mainly direct: the action of natural selection being limited to the
destruction of those who are constitutionally too feeble to live, even with
external aid. As the preservation of incapables is secured by our social
arrangements; and as very few save incarcerated criminals are prevented by
their inferiorities from leaving the average number of offspring; it
results that survival of the fittest can scarcely at all act in such way as
to produce specialities of nature, either bodily or mental. Here the
specialities of nature, chiefly mental, which we see produced, and which
are so rapidly produced that a few centuries show a considerable change,
must be ascribed almost wholly to direct equilibration.[54]

CHAPTER XIV.

THE CONVERGENCE OF THE EVIDENCES.

§ 171. Of the three classes of evidences that have been assigned in proof
of Evolution, the _à priori_, which we took first, were partly negative,
partly positive.

On considering the "General Aspects of the Special-creation hypothesis," we
discovered it to be worthless. Discredited by its origin, and wholly
without any basis of observed fact, we found that it was not even a
thinkable hypothesis; and, while thus intellectually illusive, it turned
out to have moral implications irreconcilable with the professed beliefs of
those who hold it.

Contrariwise, the "General Aspects of the Evolution-hypothesis" begot the
stronger faith in it the more nearly they were considered. By its lineage
and its kindred, it was found to be as closely allied with the proved
truths of modern science, as is the antagonist hypothesis with the proved
errors of ancient ignorance. We saw that instead of being a mere
pseud-idea, it admits of elaboration into a definite conception: so showing
its legitimacy as an hypothesis. Instead of positing a purely fictitious
process, the process which it alleges proves to be one actually going on
around us. To which add that, morally considered, this hypothesis presents
no radical incongruities.

Thus, even were we without further means of judging, there could be no
rational hesitation which of the two views should be entertained.

§ 172. Further means of judging, however, we found to be afforded by
bringing the two hypotheses face to face with the general truths
established by naturalists. These inductive evidences were dealt with in
four chapters.

"The Arguments from Classification" were these. Organisms fall into groups
within groups; and this is the arrangement which we see results from
evolution, where it is known to take place. Of these groups within groups,
the great or primary ones are the most unlike, the sub-groups are less
unlike, the sub-sub-groups still less unlike, and so on; and this, too, is
a characteristic of groups demonstrably produced by evolution. Moreover,
indefiniteness of equivalence among the groups is common to those which we
know have been evolved, and those here supposed to have been evolved. And
then there is the further significant fact, that divergent groups are
allied through their lowest rather than their highest members.

Of "the Arguments from Embryology," the first is that when developing
embryos are traced from their common starting point, and their divergences
and re-divergences symbolized by a genealogical tree, there is manifest a
general parallelism between the arrangement of its primary, secondary, and
tertiary branches, and the arrangement of the divisions and sub-divisions
of our classifications. Nor do the minor deviations from this general
parallelism, which look like difficulties, fail, on closer observation, to
furnish additional evidence; since those traits of a common ancestry which
embryology reveals, are, if modifications have resulted from changed
conditions, liable to be disguised in different ways and degrees in
different lines of descendants.

We next considered "the Arguments from Morphology." Apart from those
kinships among organisms disclosed by their developmental changes, the
kinships which their adult forms show are profoundly significant. The
unities of type found under such different externals, are inexplicable
except as results of community of descent with non-community of
modification. Again, each organism analyzed apart, shows, in the likenesses
obscured by unlikenesses of its component parts, a peculiarity which can be
ascribed only to the formation of a more heterogeneous organism out of a
more homogeneous one. And once more, the existence of rudimentary organs,
homologous with organs that are developed in allied animals or plants,
while it admits of no other rational interpretation, is satisfactorily
interpreted by the hypothesis of evolution.

Last of the inductive evidences, came "the Arguments from Distribution."
While the facts of distribution in Space are unaccountable as results of
designed adaptation of organisms to their habitats, they are accountable as
results of the competition of species, and the spread of the more fit into
the habitats of the less fit, followed by the changes which new conditions
induce. Though the facts of distribution in Time are so fragmentary that no
positive conclusion can be drawn, yet all of them are reconcilable with the
hypothesis of evolution, and some of them yield it strong support:
especially the near relationship existing between the living and extinct
types in each great geographical area.

Thus of these four groups, each furnished several arguments which point to
the same conclusion; and the conclusion pointed to by the arguments of any
one group, is that pointed to by the arguments of every other group. This
coincidence of coincidences would give to the induction a very high degree
of probability, even were it not enforced by deduction. But the conclusion
deductively reached, is in harmony with the inductive conclusion.

§ 173. Passing from the evidence that evolution has taken place, to the
question--How has it taken place? we find in known agencies and known
processes, adequate causes of its phenomena.

In astronomic, geologic, and meteorologic changes, ever in progress, ever
combining in new and more involved ways, we have a set of inorganic factors
to which all organisms are exposed; and in the varying and complicating
actions of organisms on one another, we have a set of organic factors that
alter with increasing rapidity. Thus, speaking generally, all members of
the Earth's Flora and Fauna experience perpetual re-arrangements of
external forces.

Each organic aggregate, whether considered individually or as a
continuously-existing species, is modified afresh by each fresh
distribution of external forces. To its pre-existing differentiations new
differentiations are added; and thus that lapse to a more heterogeneous
state, which would have a fixed limit were the circumstances fixed, has its
limit perpetually removed by the perpetual change of the circumstances.

These modifications upon modifications which result in evolution
structurally considered, are the accompaniments of those functional
alterations continually required to re-equilibrate inner with outer
actions. That moving equilibrium of inner actions corresponding with outer
actions, which constitutes the life of an organism, must either be
overthrown by a change in the outer actions, or must undergo perturbations
that cannot end until there is a re-adjusted balance of functions and
correlative adaptation of structures.

But where the external changes are either such as are fatal when
experienced by the individuals, or such as act on the individuals in ways
that do not affect the equilibrium of their functions; then the
re-adjustment results through the effects produced on the species as a
whole--there is indirect equilibration. By the preservation in successive
generations of those whose moving equilibria are least at variance with the
requirements, there is produced a changed equilibrium completely in harmony
with the requirements.

§ 174. Even were this the whole of the evidence assignable for the belief
that organisms have been gradually evolved, it would have a warrant higher
than that of many beliefs which are regarded as established. But the
evidence is far from exhausted.

At the outset it was remarked that the phenomena presented by the organic
world as a whole, cannot be properly dealt with apart from the phenomena
presented by each organism, in the course of its growth, development, and
decay. The interpretation of either implies interpretation of the other;
since the two are in reality parts of one process. Hence, the validity of
any hypothesis respecting the one class of phenomena, may be tested by its
congruity with phenomena of the other class. We are now about to pass to
the more special phenomena of development, as displayed in the structures
and functions of individual organisms. If the hypothesis that plants and
animals have been progressively evolved be true, it must furnish us with
keys to these phenomena. We shall find that it does this; and by doing it
gives numberless additional vouchers for its truth.

CHAPTER XIV^A.

RECENT CRITICISMS AND HYPOTHESES.

§ 174a. Since the first edition of this work was published, and more
especially since the death of Mr. Darwin, an active discussion of the
Evolution hypothesis has led to some significant results.

That organic evolution has been going on from the dawn of life down to the
present time, is now a belief almost universally accepted by zoologists and
botanists--"almost universally," I say, because the surviving influence of
Cuvier prevents acceptance of it by some of them in France. Omitting the
ideas of these, all biological interpretations, speculations, and
investigations, tacitly assume that organisms of every kind in every era
and in every region have come into existence by the process of descent with
modification.

But while concerning the fact of evolution there is agreement, concerning
its causes there is disagreement. The ideas of naturalists have, in this
respect, undergone a differentiation increasingly pronounced; which has
ended in the production of two diametrically opposed beliefs. The cause
which Mr. Darwin first made conspicuous has come to be regarded by some as
the sole cause; while, on the part of others there has been a growing
recognition of the cause which he at first disregarded but afterwards
admitted. Prof. Weismann and his supporters contend that natural selection
suffices to explain everything. Contrariwise, among many who recognize the
inheritance of functionally-produced changes, there are a few, like the
Rev. Prof. Henslow, who regard it as the sole factor.

The foregoing chapters imply that the beliefs of neither extreme are here
adopted. Agreeing with Mr. Darwin that both factors have been operative, I
hold that the inheritance of functionally-caused alterations has played a
larger part than he admitted even at the close of his life; and that,
coming more to the front as evolution has advanced, it has played the chief
part in producing the highest types. I am not now about to discuss afresh
these questions, but to deal with certain further questions.

For while there has been taking place in the biological world the major
differentiation above indicated, there have been taking place certain minor
differentiations--there have been arising special views respecting the
process of organic evolution. Concerning each of these it is needful to say
something.

§ 174b. Among the implied controversies the most conspicuous one has
concerned the alleged process called by Prof. Weismann _Panmixia_--a
process which Dr. Romanes had foreshadowed under the name of "the Cessation
of Selection." Dr. Romanes says:--"At that time it appeared to me, as it
now appears to Weismann, entirely to supersede the necessity of supposing
that the effect of disuse is ever inherited in any degree at all."[55] The
alleged mode of action is exemplified by Prof. Weismann as follows:--

  "A goose or a duck must possess strong powers of flight in the natural
  state, but such powers are no longer necessary for obtaining food when it
  is brought into the poultry-yard, so that a rigid selection of
  individuals with well-developed wings, at once ceases among its
  descendants. Hence in the course of generations, a deterioration of the
  organs of flight must necessarily ensue, and the other members and organs
  of the bird will be similarly affected."[56]

Here, and throughout the arguments of those who accept the hypothesis of
Panmixia, there is an unwarranted assumption--nay, an assumption at
variance with the doctrine in support of which it is made. It is contended
that in such cases as the one given there will, apart from any effects of
disuse, be decrease in the disused organs because, not being kept by
Natural Selection up to the level of strength previously needed, they will
vary in the direction of decrease; and that variations in the direction of
decrease, occurring in some individuals, will, by interbreeding, produce an
average decrease throughout the species. But why will the disused organs
vary in the direction of decrease more than in the direction of increase?
The hypothesis of Natural Selection postulates indeterminate
variations--deviations no more in one direction than in the opposite
direction: implying that increases and decreases of size will occur to
equal extents and with equal frequencies. With any other assumption the
hypothesis lapses; for if the variations in one direction exceed those in
another the question arises--What makes them do this? And whatever makes
them do this becomes the essential cause of the modification: the selection
of favourable variations is tacitly admitted to be an insufficient
explanation. But if the hypothesis of Natural Selection itself implies the
occurrence of equal variations on all sides of the mean, how can Panmixia
produce decrease? _Plus_ deviations will cancel _minus_ deviations, and the
organ will remain where it was.[57]

"But you have forgotten the tendency to economy of growth," will be the
reply--"you have forgotten that in Mr. Darwin's words 'natural selection is
continually trying to economize in every part of the organization;' and
that this is a constant cause favouring _minus_ variations." I have not
forgotten it; but have remembered it as showing how, to support the
hypothesis of Panmixia, there is invoked the aid of that very hypothesis
which it is to replace. For this principle of economy is but another aspect
of the principle of functionally-produced modifications. Nearly forty years
ago I contended that "the different parts of ... an individual organism
compete for nutriment; and severally obtain more or less of it according as
they are discharging more or less duty:"[58] the implication being that as
all other organs are demanding blood, decrease of duty in any one,
entailing decreased supply of blood, brings about decreased size. In other
words, the alleged economy is nothing else than the abstraction, by active
parts, of nutriment from an inactive part; and is merely another name for
functionally-produced decrease. So that if the variations are supposed to
take place predominantly in the direction of decrease, it can only be by
silently assuming the cause which is overtly denied.

But now we come to the strange fact that the particular case in which
panmixia is assigned in disproof of alleged inheritance of
functionally-produced modifications, is a case in which it would be
inapplicable even were its assumption legitimate--the case of disused
organs in domestic animals. For since nutrition is here abundant, the
principle of economy under the form alleged does not come into play.
Contrariwise, there even occurs a partial re-development of rudimentary
organs: instances named by Mr. Darwin being the supplementary mammæ in
cows, fifth toes on the hind feet of dogs, spurs and comb in hens, and
canine teeth in mares. Now clearly, if organs disused for innumerable
generations may thus vary in the direction of increase, it must, _a
fortiori_, be so with recently disused organs, and there disappears all
plea (even the illegitimate plea) for assuming that in the wing of a wild
duck which has become domesticated, the _minus_ variations will exceed the
_plus_ variations: the hypothesis of panmixia loses its postulate.

If it be said that Mr. Darwin's argument is based on the changed ratio
between the weights of leg-bones and wing-bones, and that this changed
ratio may result not from decrease of the wing-bones but from increase of
the leg-bones, then there comes a fatal reply. Such, increase cannot be
ascribed to selection of varieties, since there is no selective breeding to
obtain larger legs, and as it is not pretended that panmixia accounts for
increase the case is lost: there remains no cause for such increase save
increase of function.

§ 174c. The doctrine of determinate evolution or definitely-directed
evolution, which appears to be in one form or other entertained by sundry
naturalists, has been set forth by the late Prof. Eimer under the title
"Orthogenesis." A distinct statement of his conception is not easily made
for the reason that, as I think, the conception itself is indistinct. Here
are some extracts from a translation of his paper published at Chicago. Out
of these the reader may form a notion of the theory:

  "Orthogenesis shows that organisms develop in definite directions without
  the least regard for utility through purely physiological causes as the
  result of _organic growth_, as I term the process."

  "I am concerned in this paper with definitely directed evolution as the
  cause of transmutation, and not with the effects of the use and activity
  of organs which with Lamarck I adopted as the second main explanatory
  cause thereof."

  "The causes of definitely directed evolution are contained, according to
  my view, in the effects produced by outward circumstances and influences
  such as climate and nutrition upon the constitution of a given organism."

  "At variance with all the facts of definitely directed evolution ... is
  also the contention of my opponent [Weismann] ... that the variations
  demonstrably oscillate to and fro in the most diverse directions about a
  given zero-point. There is no oscillation in the direction of
  development, but simply an advance forwards in a straight line with
  occasional lateral divergences whereby the forkings of the ancestral tree
  are produced."[59]

These sentences contain one of those explanations which explain nothing;
for we are not enabled to see how the "outward circumstances and
influences" produce the effects ascribed to them. We are not shown in what
way they cause organic evolution in general, still less in what way they
cause the infinitely-varied forms in which organic evolution results. The
assertion that evolution takes definitely-directed lines is accompanied by
no indication of the reasons why particular lines are followed rather than
others. In short, we are simply taken a step back, and for further
interpretation referred to a cause said to be adequate, but the operations
of which we are to imagine as best we may.

This is a re-introduction of supernaturalism under a disguise. It may pair
off with the conception made popular by the _Vestiges of the Natural
History of Creation_, in which it was contended that there exists a
persistent tendency towards the birth of a higher form of creature; or it
may be bracketed with the idea entertained by the late Prof. Owen, who
alleged an "ordained becoming" of living things.

§ 174d. An objection to the Darwinian doctrine which has risen into
prominence, is that Natural Selection does not explain that which it
professes to explain. In the words of Mr. J. T. Cunningham:--

  "Everybody knows that the theory of natural selection was put forward by
  Darwin as a theory of the origin of species, and yet it is only a theory
  of the origin of adaptations. The question is: Are the differences
  between species differences of adaptation? If so, then the origin of
  species and the origin of adaptations are equivalent terms. But there is
  scarcely a single instance in which a specific character has been shown
  to be useful, to be adaptive."[60]

To illustrate this last statement Mr. Cunningham names the plaice,
flounder, and dab as three flat fishes in which, along with the adaptive
characters related to the mode of life common to them all, each has
specific characters which are not adaptive. No evidence is forthcoming that
these in any way conduce to the welfare of the species. Two propositions
are here involved which should be separately dealt with.

The first is that the adaptive modifications which survival of the fittest
is able to produce, do not become specific traits: they are traits separate
in kind from those which mark off groups proved to be specifically distinct
by their inability to breed together. Such evidence as we at present have
seems to warrant this statement. Out of the many varieties of dogs most, if
not all, have been rendered distinct by adaptive modifications, mostly
produced by selection. But, notwithstanding the immense divergences of
structure so produced, the varieties inter-breed. To this, however, it may
be replied that sufficient time has not elapsed--that the process by which
a structural adaptation so reacts on the constitution as to make it a
distinct one, possibly, or probably, takes many thousands of years. Let us
accept for the moment Lord Kelvin's low estimate of the geologic time
during which life has existed--one hundred million years. Suppose we divide
that time into as many parts as there are hours occupied in the development
of a human foetus. And suppose that during these hundred million years
there has been going on with some uniformity the evolution of the various
organic types now existing. Then the amount of change undergone by the
foetus in an hour, will be equivalent to the amount of change undergone by
an evolving organic form in fifteen thousand years. That is to say, during
general evolution it may have taken fifteen thousand years to establish, as
distinct, two species differing from one another no more than the foetus
differs from itself after the lapse of an hour. Hence, though we lack proof
that adaptive modifications become specific traits, it is quite possible
that they are in course of becoming specific traits.

The converse proposition, that the traits by which species are ordinarily
distinguished are non-adaptive traits is well sustained; and the statement
that, if not themselves useful they are correlated with those which are
useful, is, to say the least, unproved. For the instances given by Mr.
Darwin of correlated traits are not those between adaptive traits and the
traits regarded as specific, but between traits none of which are specific;
as between skull and limbs in swine, tusks and bristles in swine, horns and
wool in sheep, beak and feet in pigeons.

If we seek a clue in those processes by which correlations are brought
about--the physiological actions and reactions--we may at once see that any
organic modification, be it adaptive or not, must entail secondary
modifications throughout the rest of the organism, most of them insensible
but some of them sensible. The competition for blood among organs, referred
to above, necessitates that, other things remaining the same, the extra
growth of any one tells on all others, in variable degrees according to
conditions, and may cause appreciable diminutions of some. This is not all.
While the quantity of blood supplied to other organs is affected, its
quality also is in some cases affected. Each organ, or at any rate each
class of organs, has special nutrition--abstracts from the blood a
proportion of ingredients different from that abstracted by other organs or
classes of organs. Hence may result a deficiency or a surplus of some
element: instance the change in the blood which must be caused by growth of
a stag's antlers. Now if such effects are always produced, and if, further,
a change of general nutrition caused by a new food or by a difference of
ability to utilize certain components of food, similarly operates (instance
the above named correlation between horns and wool), then every
modification must entail throughout the organism multitudinous alterations
of structure. Such alterations will ordinarily be neither in themselves
useful nor necessarily correlated with those which are useful; since they
must arise as concomitants of any change, whether adaptive or not. There
will consequently arise the innumerable minute differences presented by
allied species in addition to the differences called specific.

On joining with recognition of this general process a recognition of the
tendency towards localization of deposit, one possible origin of specific
marks is suggested. When in an organism the circulating fluids contain
useless matter, normal or abnormal, the excretion of it, once determined
towards a certain place, continues at that place. Trees furnish examples in
the casting out of gums and resins. Animal life yields evidence in gouty
concretions and such morbid products as tubercle. A place of enfeebled
nutrition is commonly chosen--not unfrequently a place where a local injury
has occurred. Now if we extend this principle, well recognized in
pathological processes, to physiological processes, we may infer that where
an adaptive modification has so reacted on the blood as to leave some
matter to be got rid of, the deposit of this, initiated at some place of
least resistance, may produce a local structure which eventually becomes a
species-mark. A relevant inquiry suggests itself--What proportion of
species-marks are formed out of inanimate tissue or tissue of low
vitality--tissue which, like hair, feathers, horns, teeth, is composed of
by-products unfit for carrying on vital actions.

§ 174e. In the days when, not having been better instructed by Mr. Darwin,
I believed that all changes of structure in organisms result from changes
of function, I held that the cause of such changes of function is
migration. Assuming as the antecedent of migration a great geologic change,
such as upheaval of the East Indian Archipelago step by step into a
continent, it was argued, in an essay I then wrote, that, subjected
primarily to new influences in its original habitat, each kind of plant and
animal would secondarily be subjected to the altered conditions consequent
on spreading over the upheaved regions.

  "Each species being distributed over an area of some extent, and tending
  continually to colonize the new area exposed, its different members would
  be subject to different sets of changes. Plants and animals spreading
  towards the equator would not be affected in the same way with others
  spreading from it. Those spreading towards the new shores would undergo
  changes unlike the changes undergone by those spreading into the
  mountains. Thus, each original race of organisms would become the root
  from which diverged several races differing more or less from it and from
  one another."

It was further argued that, beyond modifications caused by change of
physical conditions and food, others would be caused by contact of the
Flora and Fauna of each island with the Floras and Faunas of other islands:
bringing experience of animals and plants before unknown.[61]

While this conception was wrong in so far as it ascribed the production of
new species entirely to inheritance of functionally-wrought alterations
(thus failing to recognize Natural Selection, which was not yet
enunciated), it was right in so far as it ascribed organic changes to
changes of conditions. And it was, I think, also right in so far as it
implied that isolation is a condition precedent to such changes. Apparently
it did not occur to me as needful to specify this isolation as making
possible the differentiation of species; since it goes without saying that
members of a species spreading east, west, north, south, and forming groups
hundreds of miles apart, must, while breeding with those of the same group
be prevented from breeding with those of other groups--prevented from
having their locally-caused modifications mutually cancelled.

The importance of isolation has of late been emphasized by Dr. Romanes and
others, who, to that isolation consequent on geographical diffusion, have
added that isolation which results from difference of station in the same
habitat, and also that due to differences in the breeding periods arising
in members of the same species. Doubtless in whatever way effected, the
isolation of a group subject to new conditions and in course of being
changed, is requisite as a means to permanent differentiation. Doubtless
also, as contended by Mr. Gulick and Dr. Romanes, there is a difference
between the case in which an entire species being subject to the same
conditions is throughout modified in character, thus illustrating what Mr.
Gulick calls "monotypic evolution," and the case in which different parts
of the species, leading different lives, will, if they are by any means
prevented from inter-breeding with other parts, form divergent varieties:
thus illustrating "polytypic evolution."

§ 174f. Beyond geographical and topographical isolation, there is an
isolation of another kind regarded by some as having had an important share
in organic evolution. Foreshadowed by Mr. Belt, subsequently enunciated by
Mr. Catchpool, fully thought out by Mr. Gulick, and more recently
elaborated by Dr. Romanes, "Physiological Selection" is held to account for
the genesis of marked varieties side by side with their parents. It is
contended that without the kind of isolation implied by it, variations will
be swamped by inter-crossing, and divergence prevented; but that by the aid
of this kind of isolation, a uniform species may be differentiated into two
or more species, though its members continue to live in the same area.

Facts are assigned to show that slightly unlike varieties may become unable
to inter-breed either with the parent-species or with one another. This
mutual inferiority is not of the kind we might expect. We might reasonably
suppose that when varieties had diverged widely, crossing would be
impracticable, because their constitutions had become so far unlike as to
form an unworkable mixture. But there seems evidence that the infertility
arises long before such a cause could operate, and that instead of failure
to produce a workable constitution, there is failure to produce any
constitution at all--failure to fertilize. Some change in the sexual system
is suggested as accounting for this. That a minute difference in the
reproductive elements may suffice, plants prove by the fact that when two
members of slightly-divergent varieties are fertilized by each other's
pollen, the fertility is less than if each were fertilized by the pollen of
its own variety; and where the two kinds of pollen are both used, that
derived from members of the same variety is prepotent in its effect over
that derived from members of the other variety.

The writers above named contend that variations must occur in the
reproductive organs as well as in other organs; that such variations may
produce relative infertility in particular directions; and that such
relative infertility may be the first step towards prevention of crossing
and establishment of isolation: so making possible the accumulation of such
differences as mark off new species. Without doubt we have here a
legitimate supposition and a legitimate inference. Necessarily there must
happen variations of the kind alleged, and considering how sensitive the
reproductive system is to occult influences (witness among ourselves the
frequent infertility of healthy people while feeble unhealthy ones are
fertile), it is reasonable to infer that minute and obscure alterations of
this kind may make slightly-different varieties unable to inter-breed.

Granting that there goes on this "physiological selection," we must
recognize it as one among the causes by which isolation is produced, and
the differentiating influence of natural selection in the same locality
made possible.

§ 174g. The foregoing criticisms and hypotheses do not, however, affect in
any essential way the pre-existing conceptions. If, as in the foregoing
chapters, we interpret the facts in terms of that redistribution of matter
and motion constituting Evolution at large, we shall see that the general
theory, as previously held, remains outstanding.

It is indisputable that to maintain its life an organism must maintain the
moving equilibrium of its functions in presence of environing actions. This
is a truism: overthrow of the equilibrium is death. It is a corollary that
when the environment is changed, the equilibrium of functions is disturbed,
and there must follow one of two results--either the equilibrium is
overthrown or it is re-adjusted: there is a re-equilibration. Only two
possible ways of effecting the re-adjustment exist--the direct and the
indirect. In the one case the changed outer action so alters the moving
equilibrium as to call forth an equivalent reaction which balances it. If
re-equilibration is not thus effected in the individual it is effected in
the succession of individuals. Either the species altogether disappears, or
else there disappear, generation after generation, those members of it the
equilibria of whose functions are least congruous with the changed actions
in the environment; and this is the survival of the fittest or natural
selection.

If now we persist in thus contemplating the problem as a statico-dynamical
one, we shall see that much of the discussion commonly carried on is beside
the question. The centre around which the collision of arguments has taken
place, is the question of the formation of species. But here we see that
this question is a secondary and, in a sense, irrelevant one. We are
concerned with the production of evolving and diverging organic forms; and
whether these are or are not marked off by so-called specific traits, and
whether they will or will not breed together, matters little to the general
argument. If two divisions of a species, falling into unlike conditions and
becoming re-equilibrated with them, eventually acquire the differences of
nature called specific, this is but a collateral result. The _essential_
result is the formation of divergent organic forms. The biologic
atmosphere, so to speak, has been vitiated by the conceptions of past
naturalists, with whom the identification and classification of species was
the be-all and end-all of their science, and who regarded the traits which
enabled them to mark off their specimens from one another, as the traits of
cardinal importance in Nature. But after ignoring these technical ideas it
becomes manifest that the distinctions, morphological or physiological,
taken as tests of species, are merely incidental phenomena.

Moreover, on continuing thus to look at the facts, we shall better
understand the relation between adaptive and specific characters, and
between specific characters and those many small differences which always
accompany them. For during re-equilibration there must, beyond those
changes of structure required to balance outer actions by inner actions, be
numerous minor changes. In any complex moving equilibrium alterations of
larger elements inevitably cause alterations of elements immediately
dependent on them, and these again of others: the effects reverberate and
re-reverberate throughout the entire aggregate of actions down to the most
minute. Of resulting structural changes a few will be conspicuous, more
will be less conspicuous, and so on continuously multiplying in number and
decreasing in amount.

Here seems a fit place for remarking that there are certain processes which
do not enter into these re-equilibrations but in a sense interfere with
them. One example must suffice. Among dogs may be observed the trick of
rolling on some mass having a strong animal smell: commonly a decaying
carcase. This trick has probably been derived from the trick of rolling on
the body of an animal caught and killed, and so gaining a tempting odour. A
male dog which first did this, and left a trail apt to be mistaken for that
of prey, would be more easily found by a female, and would be more likely
than others to leave posterity. Now such a trick could have no relation to
better maintenance of the moving equilibrium, and might very well arise in
a dog having no superiority. If it arose in one of the worst it would be
eliminated from the species, but if it arose in one of medium constitution,
fairly capable of self-preservation, it would tend to produce survival of
certain of the less fit rather than the fittest. Probably there are many
such minor traits which are in a sense accidental, and are neither adaptive
nor specific in the ordinary sense.

§ 174h. But now let it be confessed that though all phenomena of organic
evolution must fall within the lines above indicated, there remain many
unsolved problems.

Take as an instance the descent of the testes in the _Mammalia_. Neither
direct nor indirect equilibration accounts for this. We cannot consider it
an adaptive change, since there seems no way in which the production of
sperm-cells, internally carried on in a bird, is made external by
adjustment to the changed requirements of mammalian life. Nor can we
ascribe it to survival of the fittest; for it is incredible that any mammal
was ever advantaged in the struggle for life by this changed position of
these organs. Contrariwise, the removal of them from a place of safety to a
place of danger, would seem to be negatived by natural selection. Nor can
we regard the transposition as a concomitant of re-equilibration; since it
can hardly be due to some change in the general physiological balance.

An example of another order is furnished by the mason-wasp. Several
instincts, capacities, peculiarities, which are in a sense independent
though they cooperate to the same end, are here displayed. There is the
instinct to build a cell of grains of sand, and the ability to do this,
which though in a sense separate may be regarded as an accompaniment; and
there is the secretion of a cement--a physiological process not directly
connected with the psychological process. After oviposition there comes
into play the instinct to seek, carry home, and pack into the cell, the
small caterpillars, spiders, &c., which are to serve as food for the larva;
and then there is the instinct to sting each of them at a spot where the
injected hypnotic poison keeps the creature insensible though alive till it
is wanted. These cannot be regarded as parts of a whole developed in
simultaneous coordination. There is no direct connexion between the
building instinct and the hypnotizing instinct; still less between these
instincts and the associated appliances. What were the early stages they
passed through imagination fails to suggest. Their usefulness depends on
their combination; and this combination would seem to have been useless
until they had all reached something like their present completeness. Nor
can we in this case ascribe anything to the influence of teaching by
imitation, supposed to explain the doings of social insects; for the
mason-wasp is solitary.

Thus the process of organic evolution is far from being fully understood.
We can only suppose that as there are devised by human beings many puzzles
apparently unanswerable till the answer is given, and many necromantic
tricks which seem impossible till the mode of performance is shown; so
there are apparently incomprehensible results which are really achieved by
natural processes. Or, otherwise, we must conclude that since Life itself
proves to be in its ultimate nature inconceivable, there is probably an
inconceivable element in its ultimate workings.

END OF VOL. I.

APPENDICES.

APPENDIX A.

THE GENERAL LAW OF ANIMAL FERTILITY.

[_In the_ Westminster Review _for April, 1852, I published an essay under
the title "A Theory of Population deduced from the General Law of Animal
Fertility." That essay was the germ of Part VI of this work, "The Laws of
Multiplication," in which its essential theses are fully developed. When
developing them, I omitted some portions of the original essay--one which
was not directly relevant, and another which contained a speculation open
to criticism. As indicated in § 74f, I find that this speculation has an
unexpected congruity with recent results of inquiry. I therefore decide to
reproduce it here along with the definition of Life propounded in that
essay, which, though subsequently replaced by the definition elaborated in
Part I, contains an element of truth._]

*    *    *    *    *

Some clear idea of the nature of Life itself, must, indeed, form a needful
preliminary. We may be sure that a search for the influences determining
the maintenance and multiplication of living organisms, cannot be
successfully carried out unless we understand what is the peculiar property
of a living organism--what is the widest generalization of the phenomena
that indicate life. By way of preparation, therefore, for the Theory of
Population presently to be developed, we propose devoting a brief space to
this prior question.

*    *    *    *    *

Employing the term, then, in its usual sense, as applicable only to
organisms, Life may be defined as--_the co-ordination of actions_. The
growth of a crystal, which is the highest inorganic process we are
acquainted with, involves but one action--that of accretion. The growth of
a cell, which is the lowest organic process, involves two
actions--accretion and disintegration--repair and waste--assimilation and
oxidation. Wholly deprive a cell of oxygen, and it becomes inert--ceases to
manifest vital phenomena; or, as we say, dies. Give it no matter to
assimilate, and it wastes away and disappears, from continual oxidation.
Evidently, then, it is in the balance of these two actions that the life
consists. It is not in the assimilation alone; for the crystal assimilates:
neither is it in the oxidation alone; for oxidation is common to inorganic
matter: but it is in the joint maintenance of these--the _co-ordination_ of
them. So long as the two go on together, life continues: suspend either of
them, and the result is--death.

The attribute which thus distinguishes the lowest organic from the highest
inorganic bodies, similarly distinguishes the higher organisms from the
lower ones. It is in the greater complexity of the co-ordination--that is,
in the greater number and variety of the co-ordinated actions--that every
advance in the scale of being essentially consists. And whether we regard
the numerous vital processes carried on in a creature of complex structure
as so many additional processes, or whether, more philosophically, we
regard them as subdivisions of the two fundamental ones--oxidation and
accretion--the co-ordination of them is still the life. Thus turning to
what is physiologically classified as the _vegetative system_, we see that
stomach, lungs, heart, liver, skin, and the rest, must work in concert. If
one of them does too much or too little--that is, if the co-ordination be
imperfect--the life is disturbed; and if one of them ceases to act--that
is, if the co-ordination be destroyed--the life is destroyed. So likewise
is it with the _animal system_, which indirectly assists in co-ordinating
the actions of the viscera by supplying food and oxygen. Its component
parts, the limbs, senses, and instruments of attack or defence must perform
their several offices in proper sequence; and further, must conjointly
minister to the periodic demands of the viscera, that these may in turn
supply blood. How completely the several attributes of animal life come
within the definition, we shall best see on going through them _seriatim_.

Thus _Strength_ results from the co-ordination of actions; for it is
produced by the simultaneous contraction of many muscles and many fibres of
each muscle; and the strength is great in proportion to the number of these
acting together--that is, in proportion to the co-ordination. _Swiftness_
also, depending partly on strength, but requiring also the rapid
alternation of movements, equally comes under the expression; seeing that,
other things equal, the more quickly sequent actions can be made to follow
each other, the more completely are they co-ordinated. So, too, is it with
_Agility_; the power of a chamois to spring with safety from crag to crag
implies accurate co-ordination in the movements of many different muscles,
and a due subordination of them all to the perceptions. The definition
similarly includes _Instinct_, which consists in the uniform succession of
certain actions or series of actions after certain sensations or groups of
sensations; and that which surprises us in instinct is the accuracy with
which these compound actions respond to these compound sensations; that
is--the completeness of their co-ordination. Thus, likewise, is it with
_Intelligence_, even in its highest manifestations. That which we call
rationality is the power to combine, or co-ordinate a great number and a
great variety of complex actions for the achievement of a desired result.
The husbandman has in the course of years, by drainage and manuring, to
bring his ground into a fertile state; in the autumn he must plough,
harrow, and sow, for his next year's crop; must subsequently hoe and weed,
keep out cattle, and scare away birds; when harvest comes, must adapt the
mode and time of getting in his produce to the weather and the labour
market; he must afterwards decide when, and where, and how to sell to the
best advantage; and must do all this that he may get food and clothing for
his family. By properly coordinating these various processes (each of which
involves many others)--by choosing right modes, right times, right
quantities, right qualities, and performing his acts in right order, he
attains his end. But if he have done too little of this, or too much of
that; or have done one thing when he should have done another--if his
proceedings have been badly co-ordinated--that is, if he have lacked
intelligence--he fails.

We find, then, that _the co-ordination of actions_ is a definition of Life,
which includes alike its highest and its lowest manifestations; and not
only so, but expresses likewise the degree of Life, seeing that the Life is
high in proportion as the co-ordination is great. Proceeding upwards, from
the simplest organic cell in which there are but two interdependent
actions, on through the group in which many such cells are acting in
concert, on through the higher group in which some of these cells assume
mainly the respiratory and others the assimilative function--proceeding
still higher to organisms in which these two functions are subdivided into
many others, and in which some cells begin to act together as contractile
fibres; next to organisms in which the visceral division of labour is
carried yet further, and in which many contractile fibres act together as
muscles--ascending again to creatures that combine the movements of several
limbs and many bones and muscles in one action; and further, to creatures
in which complex impressions are followed by the complex acts we term
instinctive--and arriving finally at man, in whom not only are the separate
acts complex, but who achieves his ends by combining together an immense
number and variety of acts often extending through years--we see that the
progress is uniformly towards greater co-ordination of actions. Moreover,
this co-ordination of actions unconsciously constitutes the essence of our
common notion of life; for we shall find, on inquiry, that when we infer
the death of an animal, which does not move on being touched, we infer it
because we miss the usual co-ordination of a sensation and a motion: and we
shall also find, that the test by which we habitually rank creatures high
or low in the scale of vitality is the degree of co-ordination their
actions exhibit.

*    *    *    *    *

There remains but to notice the objection which possibly may be raised,
that the co-ordination of actions is not life, but the ability to maintain
life. Lack of space forbids going into this at length. It must suffice to
say, that life and the ability to maintain life will be found the same. We
perpetually expend the vitality we have that we may continue our vitality.
Our power to breathe a minute hence depends upon our breathing now. We must
digest during this week that we may have strength to digest next. That we
may get more food, we must use the force which the food we have eaten gives
us. Everywhere vigorous life is the strength, activity, and sagacity
whereby life is maintained; and equally in descending the scale of being,
or in watching the decline of an invalid, we see that the ebbing away of
life is the ebbing away of the ability to preserve life.[62]

[Only on now coming to re-read the definition of Life enunciated at the
commencement of this essay with the arguments used in justification of it,
does it occur to me that its essential thought ought to have been
incorporated in the definition of Life given in Part I. The idea of
co-ordination is there implied in the idea of correspondence, but the idea
of co-ordination is so cardinal a one that it should be expressed not by
implication but overtly. It is too late to make the required amendment in
the proper place, for the first part of this work is already stereotyped
and printed. Being unable to do better I make the amendment here. The
formula as completed will run:--The definite combination of heterogeneous
changes, both simultaneous and successful, _co-ordinated into_
correspondence with external co-existences and sequences.]

*    *    *    *    *

Ending here this preliminary dissertation, let us now proceed to our
special subject.

§ 1. On contemplating its general circumstances, we perceive that any race
of organisms is subject to two sets of conflicting influences. On the one
hand by natural death, by enemies, by lack of food, by atmospheric changes,
&c., it is constantly being destroyed. On the other hand, partly by the
strength, swiftness and sagacity of its members, and partly by their
fertility, it is constantly being maintained. These conflicting sets of
influences may be conveniently generalized as--the forces destructive of
race, and the forces preservative of race.

§ 2. Whilst any race continues to exist, the forces destructive of it and
the forces preservative of it must perpetually tend towards equilibrium. If
the forces destructive of it decrease, the race must gradually become more
numerous, until, either from lack of food or from increase of enemies, the
destroying forces again balance the preserving forces. If, reversely, the
forces destructive of it increase, then the race must diminish, until,
either from its food becoming relatively more abundant, or from its enemies
dying of hunger, the destroying forces sink to the level of the preserving
forces. Should the destroying forces be of a kind that cannot be thus met
(as great change of climate), the race, by becoming extinct, is removed out
of the category. Hence this is necessarily the _law of maintenance_ of all
races; seeing that when they cease to conform to it they cease to be.

Now the forces preservative of race are two--ability in each member of the
race to preserve itself, and ability to produce other members--power to
maintain individual life, and power to propagate the species. These must
vary inversely. When, from lowness of organization, the ability to contend
with external dangers is small, there must be great fertility to compensate
for the consequent mortality; otherwise the race must die out. When, on the
contrary, high endowments give much capacity of self-preservation, there
needs a correspondingly low degree of fertility. Given the dangers to be
met as a constant quantity; then, as the ability of any species to meet
them must be a constant quantity too, and as this is made up of the two
factors--power to maintain individual life and power to multiply--these
cannot do other than vary inversely.

§ 3. To show that observed phenomena harmonise with this _à priori_
principle seems scarcely needful But, though axiomatic in its character,
and therefore incapable of being rendered more certain, yet illustrations
of the conformity to it which nature everywhere exhibits, will facilitate
the general apprehension of it.

In the vegetable kingdom we find that the species consisting of simple
cells, exhibit the highest reproductive power. The yeast fungus, which in a
few hours propagates itself throughout a large mass of wort, offers a
familiar example of the extreme rapidity with which these lowly organisms
multiply. In the _Protococcus nivalis_, a microscopic plant which in the
course of a night reddens many square miles of snow, we have a like
example; as also in the minute _Algæ_, which colour the waters of stagnant
pools. The sudden appearance of green films on damp decaying surfaces, the
spread of mould over stale food, and the rapid destruction of crops by
mildew, afford further instances. If we ascend a step to plants of
appreciable size, we still find that in proportion as the organization is
low the fertility is great. Thus of the common puff-ball, which is little
more than a mere aggregation of cells, Fries says, "in a single individual
of _Reticularia maxima_, I have counted (calculated?) 10,000,000 sporules."
From this point upwards, increase of bulk and greater complexity of
structure are still accompanied by diminished reproductive power; instance
the _Macrocystis pyrifera_, a gigantic sea-weed, which sometimes attains a
length of 1500 feet, of which Carpenter remarks, "This development of the
nutritive surface takes place at the expense of the fructifying apparatus,
which is here quite subordinate."[63] And when we arrive at the
highly-organized exogenous trees, we find that not only are they many years
before beginning to bear with any abundance, but that even then they
produce, at the outside, but a few thousand seeds in a twelvemonth. During
its centuries of existence, an oak does not develop as many acorns as a
fungus does spores in a single night.

Still more clearly is this truth illustrated throughout the animal kingdom.
Though not so great as the fertility of the Protophyta, which, as Prof.
Henslow says, in some cases passes comprehension, the fertility of the
Protozoa is yet almost beyond belief. In the polygastric animalcules
spontaneous fission takes place so rapidly that "it has been calculated by
Prof. Ehrenberg that no fewer than 268 millions might be produced in a
month from a single _Paramecium_;"[64] and even this astonishing rate of
increase is far exceeded in another species, one individual of which, "only
to be perceived by means of a high magnifying power, is calculated to
generate 170 billions in four days."[65] Amongst the larger organisms
exhibiting this lowest mode of reproduction under a modified form--that of
gemmation--we see that, though not nearly so rapid as in the Infusoria, the
rate of multiplication is still extremely high. This fact is well
illustrated by the polypes; and in the apparent suddenness with which whole
districts are blighted by the Aphis (multiplying by internal gemmation), we
have a familiar instance of the startling results which the parthenogenetic
process can achieve. Where reproduction becomes occasional instead of
continuous, as it does amongst higher creatures, the fertility equally
bears an inverse ratio to the development. "The queen ant of the African
_Termites_ lays 80,000 eggs in twenty-four hours; and the common hairworm
(_Gordius_) as many as 8,000,000 in less than one day."[66] Amongst the
_Vertebrata_ the lowest are still the most prolific. "It has been
calculated," says Carpenter, "that above a million of eggs are produced at
once by a single codfish."[67] In the strong and sagacious shark
comparatively few are found. Still less fertile are the higher reptiles.
And amongst the Mammalia, beginning with small Rodents, which quickly reach
maturity, produce large litters, and several litters in the year; advancing
step by step to the higher mammals, some of which are long in attaining the
reproductive age, others of which produce but one litter in a year, others
but one young one at a time, others who unite these peculiarities; and
ending with the elephant and man, the least prolific of all, we find that
throughout this class, as throughout the rest, ability to multiply
decreases as ability to maintain individual life increases.

§ 4. The _à priori_ principle thus exemplified has an obverse of a like
axiomatic character. We have seen that for the continuance of any race of
organisms it is needful that the power of self-preservation and the power
of reproduction should vary inversely.

We shall now see that, quite irrespective of such an end to be subserved,
these powers could not do otherwise than vary inversely. In the nature of
things species can subsist only by conforming to this law; and equally in
the nature of things they cannot help conforming to it.

Reproduction, under all its forms, may be described as the separation of
portions of a parent plant or animal for the purpose of forming other
plants or animals. Whether it be by spontaneous fission, by gemmation, or
by gemmules; whether the detached products be bulbels, spores or seeds,
ovisacs, ova or spermatozoa; or however the process of multiplication be
modified, it essentially consists in the throwing off of parts of adult
organisms for the purpose of making new organisms. On the other hand, self
preservation is fundamentally a maintenance of the organism in undiminished
bulk. Amongst the lowest forms of life, aggregation of tissue is the only
mode in which the self-preserving power is shown. Even in the highest,
sustaining the body in its integrity is that in which self-preservation
most truly consists--is the end which the widest intelligence is indirectly
made to subserve. Whilst, on the one side, it cannot be denied that the
increase of tissue constituting growth is self-preservation both in essence
and in result; neither can it, on the other side, be denied that a
diminution of tissue, either from injury, disease, or old age, is in both
essence and result the reverse.

Hence the maintenance of the individual and the propagation of the race
being respectively aggregative and separative, _necessarily_ vary
inversely. Every generative product is a deduction from the parental life;
and, as already pointed out, to diminish life is to diminish the ability to
preserve life. The portion thrown off is organised matter; vital force has
been expended in the organisation of it, and in the assimilation of its
component elements; which vital force, had no such portion been made and
thrown off, _would have been available for the preservation of the parent_.

Neither of these forces, therefore, can increase, save at the expense of
the other. The one draws in and incorporates new material; the other throws
off material previously incorporated. The one adds to; the other takes
from. Using a convenient expression for describing the facts (though one
that must not be construed into an hypothesis), we may say that the force
which builds up and repairs the individual is an attractive force, whilst
that which throws off germs is a repulsive force. But whatever may turn out
to be the true nature of the two processes, it is clear that they are
mutually destructive; or, stating the proposition in its briefest
form--Individuation and Reproduction are antagonistic.

Again, illustrating the abstract by reference to the concrete, let us now
trace throughout the organic world the various phases of this antagonism.

§ 5. All the lowest animal and vegetable forms--_Protozoa_ and
_Protophyta_--consist essentially of a single cell containing fluid, and
having usually a solid nucleus. This is true of the Infusoria, the simplest
Entozoa, and the microscopic Algæ and Fungi. The organisms so constituted
uniformly multiply by spontaneous fission. The nucleus, originally
spherical, becomes elongated, then constricted across its smallest
diameter, and ultimately separates, when "its divisions," says Prof. Owen,
describing the process in the Infusoria, "seem to repel each other to
positions equidistant from each other, and from the pole or end of the body
to which they are nearest. The influence of these distinct centres of
assimilation is to divert the flow of the plasmatic fluid from a common
course through the body of the polygastrian to two special courses about
those centres. So much of the primary developmental process is renewed, as
leads to the insulation of the sphere of the influence of each assimilative
centre from that of the other by the progressive formation of a double
party wall of integument, attended by progressive separation of one party
wall from the other, and by concomitant constriction of the body of the
polygastrian, until the vibratile action of the superficial cilia of each
separating moiety severs the narrowed neck of union, and they become two
distinct individuals."[68] Similar in its general view is Dr. Carpenter's
description of the multiplication of vegetable cells, which he says divide,
"in virtue, it may be surmised, of a sort of mutual repulsion between the
two halves of the endochrome (coloured cell-contents) which leads to their
spontaneous separation."[69] Under a modified form of this process, the
cell-contents, instead of undergoing bisection, divide into numerous parts,
each of which ultimately becomes a separate individual. In some of the Algæ
"a whole brood of young cells may thus be at once generated in the cavity
of the parent-cell, which subsequently bursts and sets them free."[70] The
_Achlya prolifera_ multiplies after this fashion. Amongst the Fungi, too,
the same mode of increase is exemplified by the _Protococcus nivalis_. And
"it would appear that certain Infusoria, especially the _Kolpodinæ_,
propagate by the breaking-up of their own mass into reproductive
particles."[71]

Now in this fissiparous mode of multiplication, which "is amazingly
productive, and indeed surpasses in fertility any other with which we are
acquainted,"[72] we see most clearly the antagonism between individuation
and reproduction. We see that the reproductive process involves destruction
of the individual; for in becoming two, the parent fungus or polygastrian
must be held to lose its own proper existence; and when it breaks up into a
numerous progeny, does so still more completely. Moreover, this rapid mode
of multiplication not only destroys the individuals in whom it takes place,
but also involves that their individualities, whilst they continue, shall
be of the lowest kind. For assume a protozoon to be growing by imbibition
at a given rate, and it follows that the oftener it divides the smaller
must be the size it attains to; that is, the smaller the development of its
individuality. And a further manifestation of the same truth is seen in the
fact that the more frequent the spontaneous fission the shorter the
existence of each individual. So that alike by preventing anything beyond a
microscopic bulk being attained, by preventing the continuance of this in
its integrity beyond a few hours, and by being fatal when it occurs, this
most active mode of reproduction shows the strongest antagonism to
individual life.

§ 6. Whether or not we regard reproduction as resulting from a repulsive
force (and, as seen above, both Owen and Carpenter lean to some such view),
and whether or not we consider the formation of the individual as due to
the reverse of this--an attractive force--we cannot, on studying the
phenomena, help admitting that two opposite activities thus generalized are
at work; we cannot help admitting that the aggregative and separative
tendencies do in each case determine the respective developments of the
individual and the race. On ascending one degree in the scale of organic
life, we shall find this truth clearly exemplified.

For if these single-celled organisms which multiply so rapidly be supposed
to lose some of their separative tendency, what must be the result? They
now not only divide frequently, but the divided portions fly apart. How,
then, will a diminution of this separative tendency first show itself? May
we not expect that it will show itself in the divided portions _not_ flying
apart, but remaining near each other, and forming a group? This we find in
nature to be the first step in advance. The lowest compound organisms are
"_simple aggregations of vesicles without any definite arrangement,
sometimes united, but capable of existing separately_."[73] In these cases,
"every component cell of the aggregate mass that springs from a single
germ, being capable of existing independently of the rest, may be regarded
as a distinct individual."[74] The several stages of this aggregation are
very clearly seen in both the animal and vegetable kingdoms. In the
_Hæmatococcus binalis_, the plant producing the reddish slime seen on damp
surfaces, not only does each of the cells retain its original sphericity,
but each is separated from its neighbour by a wide interval filled with
mucus; so that it is only as being diffused through a mass of mucus common
to them all, that these cells can be held to constitute one individual. We
find, too, that "the component cells, even in the highest Algæ, are
generally separated from each other by a large quantity of mucilaginous
intercellular substance."[75] And, again, the tissue of the simpler
Lichens, "in consequence of the very slight adhesion of its component
cells, is said to be pulverulent."[76] Similarly the Protozoa, by their
feeble union, constitute the organisms next above them. Amongst the
Polygastrica there are many cases "in which the individuals produced by
fission or gemmation do not become completely detached from each
other."[77] The _Ophrydium_, for instance, "exists under the form of a
motionless jelly-like mass ... made up of millions of distinct and similar
individuals imbedded in a gelatinous connecting substance;"[78] and again,
the _Uvella_, or "grape monad," consists of a cluster "which strongly
resembles a transparent mulberry rolling itself across the field of view by
the ciliary action of its component individuals."[79] The parenchyma of the
Sponge, too, is made up of cells "each of which has the character of a
distinct animalcule, having a certain power of spontaneous motion,
obtaining and assimilating its own food, and altogether living _by_ and
_for_ itself;" and so small is the cohesion of these individual cells, that
the tissue they constitute "drains away when the mass is removed from the
water, like white of egg."[80]

Of course in proportion as the aggregate tendency leading to the formation
of these groups of monads is strong, we may expect that, other things
equal, the groups will be large. Proceeding upwards from the yeast fungus,
whose cells hold together in groups of four, five, and six,[81] there must
be found in each species of these composite organisms a size of group
determined by the strength of the aggregative tendency in that species.
Hence we may expect that, when this limit is passed, the group no longer
remains united, but divides. Such we find to be the fact. These groups of
cells undergo the same process that the cells themselves do. They increase
up to a certain point, and then multiply either by simple spontaneous
fission or by that modification of it called gemmation. The _Volvox
globator_, which is made up of a number of monads associated together in
the form of a hollow sphere, develops within itself a number of smaller
spheres similarly constituted; and after these, swimming freely in its
interior, have reached a certain size, the parent group of animalcules
bursts and sets the interior groups free. And here we may observe how this
compound individuality of the Volvox is destroyed in the act of
reproduction as the simple individuality of the monad is. Again, in the
higher forms of grouped cells, where something like organisation begins to
show itself, the aggregations are not only larger, but the separative
process, now carried on by the method of gemmation, no longer wholly
destroys the individual. And in fact, this gemmation may be regarded as the
form which spontaneous fission must assume in ceasing to be fatal; seeing
that gemmation essentially consists in the separation, not into halves, but
into a larger part and a smaller part; the larger part continuing to
represent the original individual. Thus in the common _Hydra_ or
fresh-water polype, "little bud-like processes are developed from the
external surface, which are soon observed to resemble the parent in
character, possessing a digestive sac, mouth, and tentacula; for a long
time, however, their cavity is connected with that of the parent; but at
last the communication is cut off, and the young polype quits its
attachment, and goes in quest of its own maintenance."[82]

§ 7. Progress from these forms of organisation to still higher forms is
similarly characterized by increase of the aggregative tendency or
diminution of the separative, and similarly exhibits the necessary
antagonism between the development of the individual and the increase of
the race. That process of grouping which constitutes the first step towards
the production of complex organisms, we shall now find repeated in the
formation of series of groups. Just as a diminution of the separative
tendency is shown in the aggregation of divided monads, so is a further
diminution of it shown in the aggregation of the divided groups of monads.
The first instance that occurs is afforded by the compound polypes. "Some
of the simpler forms of the composite _Hydroida_," says Carpenter, "may be
likened to a _Hydra_, whose gemmæ, instead of becoming detached, remain
permanently connected with the parent; and as these in their turn may
develop gemmæ from their own bodies, a structure of more or less
arborescent character may be produced."[83] A similar species of
combination is observable amongst the _Bryozoa_, and the compound
_Tunicata_. Every degree of union may be found amongst these associated
organisms; from the one extreme in which the individuals can exist as well
apart as together, to the other extreme in which the individuals are lost
in the general mass. Whilst each _Bryozoon_ is tolerably independent of its
neighbour, "in the compound _Hydroida_, the lives of the polypes are
subordinate to that of the polypdom."[84] Of the _Salpidæ_ and
_Pyrosomidæ_, Carpenter says:--"Although closely attached to one another,
these associated animals are capable of being separated by a smart shock
applied to the sides of the vessel in which they are swimming.... In other
species, however, the separate animals are imbedded in a gelatinous mass,"
and in one kind "there is an absolute union between the vascular systems of
the different individuals."[85]

In the same manner that with a given aggregative tendency there is a limit
to the size of groups, so is there a similarly-determined limit to the size
of series of groups; and that spontaneous fission which we have seen in
cells and groups of cells we here find repeated. In the lower _Annelida_,
for example, "after the number of segments in the body has been greatly
multiplied by gemmation, a separation of those of the posterior portion
begins to take place; a constriction forms itself about the beginning of
the posterior third of the body, in front of which the alimentary canal
undergoes a dilatation, whilst on the segment behind it a proboscis and
eyes are developed, so as to form the head of the young animal which is to
be budded off; and in due time, by the narrowing of the constriction, a
complete separation is effected."[86] Not unfrequently in the _Nais_ this
process is repeated in the young one before it becomes independent of the
parent. The higher _Annelida_ are distinguished by the greater number of
segments held in continuity; an obvious result of comparatively infrequent
fission. In the class _Myriapoda_, which stands next above, "there is no
known instance of multiplication by fission."[87] Yet even here the law may
be traced both in the number and structure of the segments. The length of
the body is still increased after birth "by gemmation from (or partial
fission of) the penultimate segment." The lower members of the class are
distinguished from the higher by the greater extent to which this gemmation
is carried. Moreover, the growing aggregative tendency is seen in the fact,
that each segment of the Julus "is formed by the coalescence of two
original segments,"[88] whilst in the _Scolopendridæ_, which are the
highest of this class, "the head, according to Mr. Newport, is composed of
eight segments, which are often consolidated into one piece;"[89] both of
which phenomena may be understood as arrests of that process of fission,
which, if allowed to go a little further, would have produced distinct
segments; and, if allowed to go further still, would have separated these
segments into groups.

§ 8. Remarking, first, how gradually this mode of multiplication
disappears--how there are some creatures that spontaneously divide or not
according to circumstances; others that divide when in danger (the several
parts being capable of growing into complete individuals); others which,
though not self-dividing, can live on in each half if artificially divided;
and others in which only one of the divided halves can live--how, again, in
the Crustaceans the power is limited to the reproduction of lost limbs; how
there are certain reptiles that can re-supply a lost tail, but only
imperfectly; and how amongst the higher _Vertebrata_ the ability to repair
small injuries is all that remains--remarking thus much, let us now, by way
of preparation for what is to follow, consider the significance of the
foregoing facts taken in connection with the definition of Life awhile
since given.

This spontaneous fission, which we have seen to be, in all cases, more or
less destructive of individual life, is simply a cessation in the
co-ordination of actions. From the single cell, the halves of whose
nucleus, instead of continuing to act together, begin to repel each other,
fly apart, establish distinct centres of assimilation, and finally cause
the cell to divide; up to the Annelidan, whose string of segments
separates, after reaching a certain length; we everywhere see the
phenomenon to be fundamentally this. The tendency to separate is the
tendency not to act together, probably arising from inability to act
together any longer; and the process of separation is the process of
ceasing to act together. How truly non-co-ordination is the essence of the
matter will be seen on observing that fission takes place more or less
rapidly, according as the co-ordinating apparatus is less or more
developed. Thus, "the capability of spontaneous division is one of the most
distinctive attributes of the acrite type of structure;"[90] the acrite
type of structure being that in which the neurine or nervous matter is
supposed to be diffused through the tissues in a molecular state, and in
which, therefore, there exists no distinct nervous or co-ordinating system.
From this point upwards the gradual disappearance of spontaneous fission is
clearly related to the gradual appearance of nerves and ganglia--a fact
well exemplified by the several grades of _Annelida_ and _Myriapoda_. And
when we remember that in the embryotic development of these classes, the
nervous system does not make its appearance until after the rest of the
organism has made great progress, we may even suspect that that coalescence
of segments characteristic of the _Myriapoda_, exhibits the co-ordinating
power of the rapidly-growing nervous system overtaking and arresting the
separative tendency; and doing this most where it (the nervous system) is
most developed, namely, in the head.

And here let us remark, in passing, how, from this point of view, we still
more clearly discern the antagonism of individuation and reproduction. We
before saw that the propagation of the race is at the expense of the
individual: in the above facts we may contemplate the obverse of this--may
see that the formation of the individual is at the expense of the race.
This combination of parts that are tending to separate and become distinct
beings--this union of many incipient minor individualities into one large
individuality--is an arrest of reproduction--a diminution in the number
produced. Either these units may part and lead independent lives, or they
may remain together and have their actions co-ordinated. Either they may,
by their diffusion, form a small, simple, and prolific race, or, by their
aggregation, a large, complex, and infertile one. But manifestly the
aggregation involves the infertility; and the fertility involves the
smallness.

§ 9. The ability to multiply by spontaneous fission, and the ability to
maintain individual life, are opposed in yet another mode. It is not in
respect of size only, but still more in respect of structure, that the
antagonism exists.

Higher organisms are distinguished from lower ones partly by bulk, and
partly by complexity. This complexity essentially consists in the mutual
dependence of numerous different organs, each subserving the lives of the
rest, and each living by the help of the rest. Instead of being made up of
many like parts, performing like functions, as the Crinoid, the Star-fish,
or the Millipede, a vertebrate animal is made up of many unlike parts,
performing unlike functions. From that initial form of a compound organism,
in which a number of minor individuals are simply grouped together, we may,
more or less distinctly, trace not only the increasing closeness of their
union, and the gradual disappearance of their individualities in that of
the mass, but the gradual assumption by them of special duties. And this
"physiological division of labour," as it has been termed, has the same
effect as the division of labour amongst men. As the preservation of a
number of persons is better secured when, uniting into a society, they
severally undertake different kinds of work, than when they are separate
and each performs for himself every kind of work; so the preservation of a
congeries of parts, which, combining into one organism, respectively assume
nutrition, respiration, circulation, locomotion, as separate functions, is
better secured than when those parts are independent, and each fulfils for
itself all these functions.

But the condition under which this increased ability to maintain life
becomes possible is, that the parts shall cease to separate. While they are
perpetually separating, it is clear that they cannot assume mutually
subservient duties. And it is further clear that the more the tendency to
separate diminishes, that is, the larger the groups that remain connected,
_the more minutely and perfectly can that subdivision of functions which we
call organization be carried out_.

Thus we see that in its most active form the ability to multiply is
antagonistic to the ability to maintain individual life, not only as
preventing increase of bulk, but also as preventing organization--not only
as preventing homogeneous co-ordination, but as preventing heterogeneous
co-ordination.

§ 10. To establish the unbroken continuity of this law of fertility, it
will be needful, before tracing its results amongst the higher animals, to
explain in what manner spontaneous fission is now understood, and what the
cessation of it essentially means. Originally, naturalists supposed that
creatures which multiply by self-division, under any of its several forms,
continue so to multiply perpetually. In many cases, however, it has
latterly been shown that they do not do this; and it is now becoming a
received opinion that they do not, and cannot, do this, in any case. A
fertilised germ appears here, as amongst higher organisms, to be the point
of departure; and that constant formation of new tissue implied in the
production of a great number of individuals by fission, seems gradually to
exhaust the germinal capacity in the same way that the constant formation
of new tissue, during the development of a single mammal, exhausts it. The
phenomena classified by Steenstrup as "Alternate Generation," and since
generalised by Professor Owen in his work "On Parthenogenesis," illustrate
this. The egg of a _Medusa_ (jellyfish) develops into a polypoid animal
called the _Strobila_. This _Strobila_ lives as the polype does, and, like
it, multiplies rapidly by gemmation. After a great number of individuals
has been thus produced, and when, as we must suppose, the germinal capacity
is approaching exhaustion, each _Strobila_ begins to exhibit a series of
constrictions, giving it some resemblance to a rouleau of coin or a pile of
saucers. These constrictions deepen; the segments gradually develop
tentacula; the terminal segment finally separates itself, and swims away in
the form of a young _Medusa_; the other segments, in succession, do the
same; and from the eggs which these _Medusæ_ produce, other like series of
polypoid animals, multiplying by gemmation, originate. In the compound
Polypes, in the _Tunicata_, in the _Trematoda_, and in the Aphis, we find
repeated, under various modifications, the same phenomenon.

Understanding then, this lowest and most rapid mode of multiplication to
consist essentially in the production of a great number of individuals from
a single germ--perceiving, further, that diminished activity of this mode
of multiplication consists essentially in the aggregation of the
germ-product into larger masses--and seeing, lastly, that the disappearance
of this mode of multiplication consists essentially in the aggregation of
the germ-product into _one_ mass--we shall be in a position to comprehend,
amongst the higher animals, that new aspect of the law, under which
increased individuation still involves diminished reproduction. Progressing
from those lowest forms of life in which a single ovum originates countless
organisms, through the successive stages in which the number of organisms
so originated becomes smaller and smaller; and finally arriving at a stage
in which one ovum produces but one organism; we have now, in our further
ascent, to observe the modified mode in which this same necessary
antagonism between the ability to multiply, and the ability to preserve
individual life, is exhibited.

§ 11. Throughout both the animal and vegetable kingdoms, generation is
effected "by the union of the contents of a 'sperm-cell' with those of a
'germ-cell;' the latter being that from within which the embryo is evolved,
whilst the former supplies some material or influence necessary to its
evolution."[91] Amongst the lowest vegetable organisms, as in the
_Desmideæ_, the _Diatomaceæ_, and other families of the inferior _Algæ_,
those cells do not appreciably differ; and the application to them of the
terms "sperm-cell" and "germ-cell" is hypothetical. From this point
upwards, however, distinctions become visible. As we advance to higher and
higher types of structure, marked differences arise in the character of
these cells, in the organs evolving them, and in the position of these
organs, which are finally located in separate sexes. Doubtless a separation
in the _functions_ of "sperm-cell" and "germ-cell" has simultaneously
arisen. That change from homogeneity of function to heterogeneity of
function which essentially constitutes progress in organization may be
assumed to take place here also; and, indeed, it is probable that the
distinction gradually established between these cells, in origin and
appearance, is merely significant of, and consequent upon, the distinction
that has arisen between them in constitution and office. Let us now inquire
in what this distinction consists.

If the foundation of every new organism be laid by the combination of two
elements, we may reasonably suspect that these two elements are typical of
some two fundamental divisions of which the new organism is to consist. As
nothing in nature is without meaning and purpose, we may be sure that the
universality of this binary origin, signifies the universality of a binary
structure. The simplest and broadest division of which an organism is
capable must be that signified. What, then, must this division be?

The proposed definition of organic life supplies an answer. If organic life
be the co-ordination of actions, then an organism may be primarily divided
into parts whose actions are co-ordinated, and parts which co-ordinate
them--organs which are made to work in concert, and the apparatus which
makes them so work--or, in other words, the assimilative, vascular,
excretory, and muscular systems on the one hand, and the nervous system on
the other. The justness of this classification will become further
apparent, when it is remembered that by the nervous system alone is the
individuality established. By it all parts are made one in purpose, instead
of separate; by it the organism is rendered a conscious whole--is enabled
to recognise its own extent and limits; and by it are all injuries
notified, repairs directed, and the general conservation secured. The more
the nervous system is developed, the more reciprocally subservient do the
components of the body become--the less can they bear separating. And that
which thus individuates many parts into one whole, must be considered as
more broadly distinguished from the parts individuated, than any of these
parts from each other. Further evidence in support of this position may be
drawn from the fact, that as we ascend in the scale of animal life, that
is, as the co-ordination of actions becomes greater, we find the
co-ordinating or nervous system becoming more and more definitely separated
from the rest; and in the vertebrate or highest type of structure we find
the division above insisted on distinctly marked. The co-ordinating parts
and the parts co-ordinated are placed on opposite sides of the vertebral
column. With the exception of a few ganglia, the whole of the nervous
masses are contained within the neural arches of the vertebræ; whilst all
the viscera and limbs are contained within, or appended to, the hæmal
arches--the terms neural and hæmal having, indeed, been chosen to express
this fundamental division.