INDIRECT EQUILIBRATION.
§ 164. Besides those perturbations produced in any organism by special
disturbing forces, there are ever going on many others--the reverberating
effects of disturbing forces previously experienced by the individual, or
by ancestors; and the multiplied deviations of function so caused imply
multiplied deviations of structure. In § 155 there was re-illustrated the
truth, set forth at length when treating of Adaptation (§ 69), that an
organism in a state of moving equilibrium, cannot have extra function
thrown on any organ, and extra growth produced in such organ, without
correlative changes being entailed throughout all other functions, and
eventually throughout all other organs. And when treating of Variation
(§ 90), we saw that individuals which have been made, by their different
circumstances, to deviate functionally and structurally from the average
type in different directions, will bequeath to their joint offspring,
compound perturbations of function and compound deviations of structure,
endlessly varied in their kinds and amounts.
Now if the individuals of a species are thus necessarily made unlike in
countless ways and degrees--if in one individual the amount of energy in a
particular direction is greater than in any other individual, or if here a
peculiar combination gives a resulting action which is not found elsewhere;
then, among all the individuals, some will be less liable than others to
have their equilibria overthrown by a particular incident force previously
unexperienced. Unless the change in the environment is so violent as to be
universally fatal to the species, it must affect more or less differently
the slightly-different moving equilibria which the members of the species
present. Inevitably some will be more stable than others when exposed to
this new or altered factor. That is to say, those individuals whose
functions are most out of equilibrium with the modified aggregate of
external forces, will be those to die; and those will survive whose
functions happen to be most nearly in equilibrium with the modified
aggregate of external forces.
But this survival of the fittest[52] implies multiplication of the fittest.
Out of the fittest thus multiplied there will, as before, be an
overthrowing of the moving equilibrium wherever it presents the least
opposing force to the new incident force. And by the continual destruction
of the individuals least capable of maintaining their equilibria in
presence of this new incident force, there must eventually be reached an
altered type completely in equilibrium with the altered conditions.
§ 165. This survival of the fittest, which I have here sought to express in
mechanical terms, is that which Mr. Darwin has called "natural selection,
or the preservation of favoured races in the struggle for life." That there
goes on a process of this kind throughout the organic world, Mr. Darwin's
great work on the _Origin of Species_ has shown to the satisfaction of
nearly all naturalists. Indeed, when once enunciated, the truth of his
hypothesis is so obvious as scarcely to need proof. Though evidence may be
required to show that natural selection accounts for everything ascribed to
it, yet no evidence is required to show that natural selection has always
been going on, is going on now, and must ever continue to go on.
Recognizing this as an _à priori_ certainty, let us contemplate it under
its two distinct aspects.
That organisms which live, thereby prove themselves fit for living, in so
far as they have been tried, while organisms which die, thereby prove
themselves in some respects unfitted for living, are facts no less manifest
than is the fact that this self-purification of a species must tend ever to
insure adaptation between it and its environment. This adaptation may be
either so _maintained_ or so _produced_. Doubtless many who have looked at
Nature with philosophic eyes, have observed that death of the worst and
multiplication of the best, tends towards maintenance of a constitution in
harmony with surrounding circumstances. That the average vigour of any race
would be diminished did the diseased and feeble habitually survive and
propagate; and that the destruction of such, through failure to fulfil some
of the conditions to life, leaves behind those which are able to fulfil the
conditions to life, and thus keeps up the average fitness to the conditions
of life; are almost self-evident truths. But to recognize "Natural
Selection" as a means of preserving an already-established balance between
the powers of a species and the forces to which it is subject, is to
recognize only its simplest and most general mode of action. It is the more
special mode of action with which we are here concerned. This more special
mode of action, Mr. Darwin has been the first to recognize as an
all-important factor, though, besides his co-discoverer Mr. A. R. Wallace,
some others have perceived that such a factor is at work. To him we owe due
appreciation of the fact that natural selection is capable of _producing_
fitness between organisms and their circumstances. He has worked up an
enormous mass of evidence showing that this "preservation of favoured races
in the struggle for life," is an ever-acting cause of divergence among
organic forms. He has traced out the involved results of the process with
marvellous subtlety. He has shown how hosts of otherwise inexplicable
facts, are accounted for by it. In brief, he has proved that the cause he
alleges is a true cause; that it is a cause which we see habitually in
action; and that the results to be inferred from it are in harmony with the
phenomena which the Organic Creation presents, both as a whole and in its
details. Let us glance at a few of the more important interpretations which
the hypothesis furnishes.
A soil possessing some ingredient in unusual quantity, may supply to a
plant an excess of the matter required for certain of its tissues; and may
cause all the parts formed of such tissues to be abnormally developed.
Suppose that among these are the hairs clothing its surfaces, including
those which grow on its seeds. Thus furnished with somewhat longer fibres,
its seeds, when shed, are carried a little further by the wind before they
fall to the ground. The plants growing from them, being rather more widely
dispersed than those produced by other individuals of the same species,
will be less liable to smother one another; and a greater number may
therefore reach maturity and fructify. Supposing the next generation
subject to the same peculiarity of nutrition, some of the seeds borne by
its members will not simply inherit this increased development of hairs,
but will carry it further; and these, still more advantaged in the same way
as before, will, on the average, have still more numerous chances of
continuing the race. Thus, by the survival, generation after generation, of
those possessing these longer hairs, and the inheritance of successive
increments of growth in the hairs, there may result a seed deviating
greatly from the original. Other individuals of the same species, subject
to the different physical conditions of other localities, may develop
somewhat thicker or harder coatings to their seeds: so rendering their
seeds less digestible by the birds which devour them. Such thicker-coated
seeds, by escaping undigested more frequently than thinner-coated ones,
will have additional chances of growing and leaving offspring; and this
process, acting in a cumulative manner season after season, will produce a
seed diverging in another direction from the ancestral type. Again,
elsewhere, some modification in the physiologic actions of the plant may
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.