DIRECT EQUILIBRATION.
§ 159. Every change is towards a balance of forces; and of necessity can
never cease until a balance of forces is reached. When treating of
equilibration under its general aspects (_First Principles_, Part II.,
Chap. xxii.), we saw that every aggregate having compound movements tends
continually towards a moving equilibrium; since any unequilibrated force to
which such an aggregate is subject, if not of a kind to overthrow it
altogether, must continue modifying its state until an equilibrium is
brought about. And we saw that the structure simultaneously reached must be
"one presenting an arrangement of forces that counterbalance all the forces
to which the aggregate is subject;" since, "so long as there remains a
residual force in any direction--be it excess of a force exercised by an
aggregate on its environment, or of a force exercised by its environment on
the aggregate, equilibrium does not exist; and therefore the
re-distribution of matter must continue."
It is essential that this truth should here be fully comprehended; and to
the end of insuring clear comprehension of it, some re-illustration is
desirable. The case of the Solar System will best serve our purpose. An
assemblage of bodies, each of which has its simple and compound motions
that severally alternate between two extremes, and the whole of which has
its involved perturbations, that now increase and now decrease, is here
presented to us. Suppose a new factor were brought to bear on this moving
equilibrium--say by the arrival of some wandering mass, or by an additional
momentum given to one of the existing masses--what would be the result? If
the strange body or the extra energy were very large, it might so derange
the entire system as to cause its collapse. But what if the incident
energy, falling on the system from without, proved insufficient to
overthrow it? There would then arise a set of perturbations which would, in
the course of an enormous period, slowly work round into a modified moving
equilibrium. The effects primarily impressed on the adjacent masses, and in
a smaller degree on the remoter masses, would presently become complicated
with the secondary effects impressed by the disturbed masses on one
another; and these again with tertiary effects. Waves of perturbation would
continue to be propagated throughout the entire system; until, around a new
centre of gravity, there had been established a set of planetary motions
different from the preceding ones. The new energy must gradually be used up
in overcoming the energies resisting the divergence it generates; which
antagonizing energies, when no longer opposed, set up a counter-action,
ending in a compensating divergence in the opposite direction, followed by
a re-compensating divergence, and so on. Now though instead of being, like
the Solar System, in a state of _independent_ moving equilibrium, an
organism is in a state of _dependent_ moving equilibrium (_First
Principles_, § 170); yet this does not prevent the manifestation of the
same law. Every animal daily obtains from without, a supply of energy to
replace the energy it expends; but this continual giving to its parts a new
momentum, to make up for the momentum continually lost, does not interfere
with the carrying on of actions and reactions like those just described.
Here, as before, we have a definitely-arranged aggregate of parts, called
organs, having their definitely-established actions and reactions, called
functions. These rhythmical actions or functions, and the various compound
rhythms resulting from their combinations, are so adjusted as to balance
the actions to which the organism is subject: there is a constant or
periodic genesis of energies which, in their kinds, amounts, and
directions, suffice to antagonize the energies the organism has constantly
or periodically to bear. If, then, there exists this moving equilibrium
among a set of internal actions, exposed to a set of external actions, what
must result if any of the external actions are changed? Of course there is
no longer an equilibrium. Some energy which the organism habitually
generates, is too great or too small to balance some incident energy; and
there arises a residual energy exerted by the environment on the organism,
or by the organism on the environment. This residual or unbalanced energy,
of necessity expends itself in producing some change of state in the
organism. Acting directly on some organ and modifying its function, it
indirectly modifies dependent functions and remotely influences all the
functions. As we have already seen (§§ 68, 69), if this new energy is
permanent, its effects must be gradually diffused throughout the entire
system; until it has come to be equilibrated in producing those structural
rearrangements whence result a counter-balancing energy.
The bearing of this general truth on the question we are now dealing with
is obvious. Those modifications upon modifications, which the unceasing
mutations of their environments have been all along generating in
organisms, have been in each case modifications involved by the
establishment of a new balance with the new combination of actions. In
every species throughout all geologic time, there has been perpetually
going on a rectification of the equilibrium, which has been perpetually
disturbed by the alteration of its circumstances; and every further
heterogeneity has been the addition of a structural change entailed by a
new equilibration, to the structural changes entailed by previous
equilibrations. There can be no other ultimate interpretation of the
matter, since change can have no other goal.
This equilibration between the functions of an organism and the actions in
its environment, may be either direct or indirect. The new incident force
may either immediately call forth some counteracting force, and its
concomitant structural change; or it may be eventually balanced by some
otherwise-produced change of function and structure. These two processes of
equilibration are quite distinct, and must be separately dealt with. We
will devote this chapter to the first of them.
§ 160. Direct equilibration is that process currently known as
_adaptation_. We have already seen (Part II., Chap, v.), that individual
organisms become modified when placed in new conditions of life--so
modified as to re-adjust the powers to the requirements; and though there
is great difficulty in disentangling the evidence, we found reason for
thinking (§ 82) that structural changes thus caused by functional changes
are inherited. In the last chapter, it was argued that if, instead of the
succession of individuals constituting a species, there were a
continuously-existing individual, any functional and structural divergence
produced by a new incident action, would increase until the new incident
action was counterpoised; and that the replacing of a continuously-existing
individual by a succession of individuals, each formed out of the modified
substance of its predecessor, will not prevent the like effect from being
produced. Here we further find that this limit towards which any such
organic change advances, in the species as in the individual, is a new
moving equilibrium adjusted to the new arrangement of external forces.
But now what are the conditions under which alone, direct equilibration can
occur? Are all the modifications that serve to re-fit organisms to their
environments, directly adaptive modifications? And if otherwise, which are
the directly adaptive and which are not? How are we to distinguish between
them?
There can be no direct equilibration with an external agency which, if it
acts at all, acts fatally; since the organism to be adapted disappears.
Conversely, some inaccessible benefit which a small modification in the
organism would make accessible, cannot by its action tend to produce this
modification: the modification and the benefit do not stand in dynamic
relation. The only new incident forces which can work the changes of
function and structure required to bring any animal or plant into
equilibrium with them, are such incident forces as operate on this animal
or plant, either continuously or frequently. They must be capable of
appreciably changing that set of complex rhythmical actions and reactions
constituting the life of the organism; and yet must not usually produce
perturbations that are fatal. Let us see what are the limits to direct
equilibration hence arising.
§ 161. In plants, organs engaged in nutrition, and exposed to variations in
the amounts and proportions of matters and forces utilized in nutrition,
may be expected to undergo corresponding variations. We find evidence that
they do this. The "changes of habit" which are common in plants, when taken
to places unlike in climate or soil to those before inhabited by them, are
changes of parts in which the modified external actions directly produce
modified internal actions. The characters of the stem and shoots as woody
or succulent, erect or procumbent; of the leaves in respect of their sizes,
thicknesses, and textures; of the roots in their degrees of development and
modes of growth; are obviously in immediate relation to the characters of
the environment. A permanent difference in the quantity of light or heat
affects, day after day, the processes going on in the leaves. Habitual rain
or drought alters all the assimilative actions, and appreciably influences
the organs that carry them on. Some particular substance, by its presence
in the soil, gives new qualities to some of the tissues; causing greater
rigidity or flexibility, and so affecting the general aspect. Here then we
have changes towards modified sets of functions and structures, in
equilibrium with modified sets of external forces.
But now let us turn to other classes of organs possessed by plants--organs
which are not at once affected in their actions by variations of incident
forces. Take first the organs of defence. Many plants are shielded against
animals that would else devour them, by formidable thorns; and others, like
the nettle, by stinging hairs. These must be counted among the appliances
by which equilibrium is maintained between the actions in the organism and
the actions in its environment; seeing that were these defences absent, the
destruction by herbivorous animals would be so much increased, that the
number of young plants annually produced would not suffice, as it now does,
to balance the mortality, and the species would disappear. But these
defensive appliances, though they aid in maintaining the balance between
inner and outer actions, cannot have been directly called forth by the
outer actions which they serve to neutralize; for these outer actions do
not continuously affect the functions of the plant even in a general way,
still less in the special way required. Suppose a species of nettle bare of
poison-hairs, to be habitually eaten by some mammal intruding on its
habitat. The actions of this mammal would have no direct tendency to
develop poison-hairs in the plant; since the individuals devoured could not
bequeath changes of structure, even were the actions of a kind to produce
fit ones; and since the individuals which perpetuated themselves would be
those on which the new incident force had not fallen. Organs of another
class, similarly circumstanced, are those of reproduction. Like the organs
of defence these are not, during the life of the individual plant, variably
exercised by variable external actions; and therefore do not fulfil those
conditions under which structural changes may be directly caused by changes
in the environment. The generative apparatus contained in every flower acts
only once during its existence; and even then, the parts subserve their
ends in a passive rather than an active way. Functionally-produced
modifications are therefore out of the question. If a plant's anthers are
so placed that the insect which most commonly frequents its flowers, must
come in contact with the pollen, and fertilize with it other flowers of the
same species; and if this insect, dwindling away or disappearing from the
locality, leaves behind no insects having such shapes and habits as cause
them to do the same thing efficiently, but only some which do it
inefficiently; it is clear that this change of its conditions has no
immediate tendency to work in the plant any such structural change as shall
bring about a new balance with its conditions. For the anthers, which, even
when they discharge their functions, do it simply by standing in the way of
the insect, are, under the supposed circumstances, left untouched by the
insect; and this remaining untouched cannot have the effect of so modifying
the stamens as to bring the anthers into a position to be touched by some
other insect. Only those individuals whose parts of fructification so far
differed from the average form that some other insect could serve them as
pollen-carrier, would have good chances of perpetuating themselves. And on
their progeny, inheriting the deviation, there would act no external force
directly tending to make the deviation greater; since the new circumstances
to which re-adaptation is required, are such as do not in the least alter
the equilibrium of functions constituting the life of the individual plant.
§ 162. Among animals, adaptation by direct equilibration is similarly
traceable wherever, during the life of the individual, an external change
generates some constant or repeated change of function. This is
conspicuously the case with such parts of an animal as are immediately
exposed to diffused influences, like those of climate, and with such parts
of an animal as are occupied in its mechanical actions on the environment.
Of the one class of cases, the darkening of the skin which follows exposure
to one or other extreme of temperature, may be taken as an instance; and
with the other class of cases we are made familiar by the increase and
decrease which use and disuse cause in the organs of motion. It is needless
here to exemplify these: they were treated of in the Second Part of this
work.
But in animals, as in plants, there are many indispensable offices
fulfilled by parts between which and the external conditions they respond
to, there is no such action and reaction as can directly produce an
equilibrium. This is especially manifest with dermal appendages. Some
ground exists for the conclusion that the greater or less development of
hairs, is in part immediately due to increase or decrease of demand on the
passive function, as forming a non-conducting coat; but be this as it may,
it is impossible that there can exist any such cause for those immense
developments of hairs which we see in the quills of the porcupine, or those
complex developments of them known as feathers. Such an enamelled armour as
is worn by _Lepidosteus_, is inexplicable as a direct result of any
functionally-worked change. For purposes of defence, such an armour is as
needful, or more needful, for hosts of other fishes; and did it result from
any direct reaction of the organism against any offensive actions it was
subject to, there seems no reason why other fishes should not have
developed similar protective coverings. Of sundry reproductive appliances
the like may be said. The secretion of an egg-shell round the substance of
an egg, in the oviduct of a bird, is quite inexplicable as a consequence of
some functionally-wrought modification of structure, immediately caused by
some modification of external conditions. The end fulfilled by the
egg-shell, is that of protecting the contained mass against certain slight
pressures and collisions, to which it is liable during incubation. How, by
any process of direct equilibration, could it come to have the required
thickness? or, indeed, how could it come to exist at all? Suppose this
protective envelope to be too weak, so that some of the eggs a bird lays
are broken or cracked. In the first place, the breakages or crackings are
actions which cannot react on the maternal organism in such ways as to
cause the secretion of thicker shells for the future: to suppose that they
can, is to suppose that the bird understands the cause of the evil, and
that the secretion of thicker shells can be effected by its will. In the
second place, such developing chicks as are contained in the shells which
crack or break, are almost certain to die; and cannot, therefore, acquire
appropriately-modified constitutions: even supposing any relation could
exist between the impression received and the change required. Meanwhile,
such eggs as escape breakage are not influenced at all by the requirement;
and hence, on the birds developed from them, there cannot have acted any
force tending to work the needful adjustment of functions. In no way,
therefore, can a direct equilibration between constitution and conditions
be here produced. Even in organs that can be modified by certain incident
actions into correspondence with such incident actions, there are some
re-adjustments which cannot be effected by direct balancing. It is thus
with the bones. The majority of the bones have to resist muscular strains;
and variations in the muscular strains call forth, by reaction, variations
in the strengths of the bones. Here there is direct equilibration. But
though the greater massiveness acquired by bones subject to greater
strains, may be ascribed to counter-acting forces evoked by forces brought
into action; it is impossible that the acquirement of greater lengths by
bones can be thus accounted for. It has been supposed that the elongation
of the metatarsals in wading birds, has resulted from direct adaptation to
conditions of life. To justify this supposition, however, it must be shown
that the mechanical actions and reactions in the legs of a wading bird,
differ from those in the legs of other birds; and that the differential
actions are equilibrated by the extra lengths. There is not the slightest
evidence of this. The metatarsals of a bird have to bear no appreciable
strains but those due to the superincumbent weight. Standing in the water
does not appreciably alter such strains; and even if it did, an increase in
the lengths of these bones would not fit them any better to meet the
altered strains.
§ 163. The conclusion at which we arrive is, then, that there go on in all
organisms, certain changes of function and structure that are directly
consequent on changes in the incident forces--inner changes by which the
outer changes are balanced, and the equilibrium restored. Such
re-equilibrations, which are often conspicuously exhibited in individuals,
we have reason to believe continue in successive generations; until they
are completed by the arrival at structures fitted to the modified
conditions. But, at the same time, we see that the modified conditions to
which organisms may be adapted by direct equilibration, are conditions of
certain classes only. That a new external action may be met by a new
internal action, it is needful that it shall either continuously or
frequently be borne by the individuals of the species, without killing or
seriously injuring them; and shall act in such way as to affect their
functions. And we find that many of the environing agencies--evil or
good--to which organisms have to be adjusted, are not of these kinds: being
agencies which either do not immediately affect the functions at all, or
else affect them in ways that prove fatal.
Hence there must be at work some other process which equilibrates the
actions of organisms with the actions they are exposed to. Plants and
animals that continue to exist, are necessarily plants and animals whose
powers balance the powers acting on them; and as their environments change,
the changes which plants and animals undergo must necessarily be changes
towards re-establishment of the balance. Besides direct equilibration,
there must therefore be an indirect equilibration. How this goes on we have
now to inquire.