INTERNAL FACTORS.
§ 153. We saw at the outset (§§ 10-16), that organic matter is built up of
molecules so unstable, that the slightest variation in their conditions
destroys their equilibrium, and causes them either to assume altered
structures or to decompose. But a substance which is beyond all others
changeable by the actions and reactions of the forces liberated from
instant to instant within its own mass, must be a substance which is beyond
all others changeable by the forces acting on it from without. If their
composition fits organic aggregates for undergoing with special facility
and rapidity those re-distributions of matter and motion whence result
individual organization and life; then their composition must make them
similarly apt to undergo those permanent re-distributions of matter and
motion which are expressed by changes of structure, in correspondence with
permanent re-distributions of matter and motion in their environments.
In _First Principles_, when considering the phenomena of Evolution at
large, the leading characters and causes of those changes which constitute
organic evolution were briefly traced. Under each of the derivative laws of
force to which the passage from an incoherent, indefinite homogeneity to a
coherent, definite heterogeneity, conforms, were given illustrations drawn
from the metamorphoses of living bodies. Here it will be needful to
contemplate the several resulting processes as going on at once, in both
individuals and species.
§ 154. Our postulate being that organic evolution in general commenced with
homogeneous organic matter, we have first to remember that the state of
homogeneity is an unstable state (_First Principles_, § 149). In any
aggregate "the relations of outside and inside, and of comparative nearness
to neighbouring sources of influence, imply the reception of influences
that are unlike in quantity, or quality, or both; and it follows that
unlike changes will be produced in the parts thus dissimilarly acted upon."
Further, "if any given whole, instead of being absolutely uniform
throughout, consists of parts distinguishable from one another--if each of
these parts, while somewhat unlike other parts, is uniform within itself;
then, each of them being in unstable equilibrium, it follows that while the
changes set up within it must render it multiform, they must at the same
time render the whole more multiform than before;" and hence, "whether that
state with which we commence be or be not one of perfect homogeneity, the
process must equally be towards a relative heterogeneity." This loss of
homogeneity which the special instability of organic aggregates fits them
to display more promptly and variously than any other aggregates, must be
shown in more numerous ways in proportion as the incident forces are more
numerous. Every differentiation of structure being a result of some
difference in the relations of the parts to the agencies acting on them, it
follows that the more multiplied and more unlike the agencies, the more
varied must be the differentiations wrought. Hence the change from a state
of homogeneity to a state of heterogeneity, will be marked in proportion as
the environing actions to which the organism is supposes it is only are
complex. This transition from a uniform to a multiform state, must continue
through successive individuals. Given a series of organisms, each of which
is developed from a portion of a preceding organism, and the question is
whether, after exposure of the series for a million years to changed
incident forces, one of its members will be the same as though the incident
forces had only just changed. To say that it will, is implicitly to deny
the persistence of force. In relation to any cause of divergence, the whole
series of such organisms may be considered as fused together into a
continuously-existing organism; and when so considered, it becomes manifest
that a continuously-acting cause will go on working a
continuously-increasing effect, until some counteracting cause prevents any
further effect.
But now if any primordial organic aggregate must, in itself and through its
descendants, gravitate from uniformity to multiformity, in obedience to the
more or less multiform forces acting on it; what must happen if these
multiform forces are themselves undergoing slow variations and
complications? Clearly the process, ever-advancing towards a temporary
limit but ever having its limit removed, must go on unceasingly. On those
structural changes wrought in the once homogeneous aggregate by an original
set of incident forces, will be superposed further changes wrought by a
modified set of incident forces; and so on throughout all time. Omitting
for the present those circumstances which check and qualify its
consequences, the instability of the homogeneous must be recognized as an
ever-acting cause of organic evolution, as of all other evolution.
While it follows that every organism, considered as an individual and as
one of a series, tends thus to pass into a more heterogeneous state; it
also follows that every species, considered as an aggregate of individuals,
tends to do the like. Throughout the area it inhabits, the conditions can
never be absolutely uniform: its members must, in different parts of the
area, be exposed to different sets of incident forces. Still more decided
must this difference of exposure be when its members spread into other
habitats. Those expansive and repressive energies which set to each species
a limit that perpetually oscillates from side to side of a certain mean,
are, as we lately saw, frequently changed by new combinations of the
external factors--astronomic, geologic, meteorologic, and organic. Hence
there from time to time arise lines of diminished resistance, along which
the species flows into new localities. Such portions of the species as thus
migrate, are subject to circumstances unlike its previous average
circumstances. And from multiformity of the circumstances, must come
multiformity of the species.
Thus the law of the instability of the homogeneous has here a three-fold
corollary. As interpreted in connexion with the ever-progressing,
ever-complicating changes in external factors, it involves the conclusion
that there is a prevailing tendency towards greater heterogeneity in all
kinds of organisms, considered both individually and in successive
generations; as well as in each assemblage of organisms constituting a
species; and, by consequence, in each genus, order, and class.
§ 155. When considering the causes of evolution in general, we further saw
(_First Principles_, § 156), that the multiplication of effects aids
continually to increase that heterogeneity into which homogeneity
inevitably lapses. It was pointed out that since "the several parts of an
aggregate are differently modified by any incident force;" and since "by
the reactions of the differently modified parts the incident force itself
must be divided into differently modified parts;" it follows that "each
differentiated division of the aggregate thus becomes a centre from which a
differentiated division of the original force is again diffused. And since
unlike forces must produce unlike results, each of these differentiated
forces must produce, throughout the aggregate, a further series of
differentiations." To this it was added that, in proportion as the
heterogeneity increases, the complications arising from this multiplication
of effects grow more marked; because the more strongly contrasted the parts
of an aggregate become, the more different must be their reactions on
incident forces, and the more unlike must be the secondary effects which
these initiate; and because every increase in the number of unlike parts
adds to the number of such differentiated incident forces, and such
secondary effects.
How this multiplication of effects conspires, with the instability of the
homogeneous, to work an increasing multiformity of structure in an
organism, was shown at the time; and the foregoing pages contain further
incidental illustrations. In § 69 it was pointed out that a change in one
function must produce ever-complicating perturbations in other functions;
and that, eventually, all parts of the organism must be modified in their
states. Suppose that the head of a bison becomes much heavier, what must be
the indirect results? The muscles of the neck are put to greater exertions;
and its vertebræ have to bear additional tensions and pressures, caused
both by the increased weight of the head, and by the stronger contractions
of the muscles that support and move it. These muscles also affect their
special attachments: several of the dorsal spines suffer augmented strains;
and the vertebræ to which they are fixed are more severely taxed. Further,
this heavier head and the more massive neck it necessitates, require a
stronger fulcrum: the whole thoracic arch, and the fore-limbs which support
it, are subject to greater continuous stress and more violent occasional
shocks. And the required strengthening of the fore-quarters cannot take
place without the centre of gravity being changed, and the hind limbs being
differently reacted upon during locomotion. Any one who compares the
outline of the bison with that of its congener, the ox, will see how
profoundly a heavier head affects the entire osseous and muscular systems.
Besides this multiplication of mechanical effects, there is a
multiplication of physiological effects. The vascular apparatus is modified
throughout its whole structure by each considerable modification in the
proportions of the body. Increase in the size of any organ implies a
quantitative, and often a qualitative, reaction on the blood; and thus
alters the nutrition of all other organs. Such physiological correlations
are exemplified in the many differences which accompany difference of sex.
That the minor sexual peculiarities are brought about by the physiological
actions and reactions, is shown both by the fact that they are commonly but
faintly marked until the fundamentally distinctive organs are developed,
and that when the development of these is prevented, the minor sexual
peculiarities do not arise. No further proof is, I think, needed, that in
any individual organism or its descendants, a new external action must,
besides the primary internal change which it works, work many secondary
changes, as well as tertiary changes still more multiplied. That tendency
towards greater heterogeneity which is given to an organism by disturbing
its environment, is helped by the tendency which every modification has to
produce other modifications--modifications which must become more numerous
in proportion as the organism becomes more complex. Lastly, among the
indirect and involved manifestations of this tendency, we must not omit the
innumerable small irregularities of structure which result from the
crossing of dissimilarly-modified individuals. It was shown (§§ 89, 90)
that what are called "spontaneous variations," are interpretable as results
of miscellaneously compounding the changes wrought in different lines of
ancestors by different conditions of life. These still more complex and
multitudinous effects so produced, are further illustrations of the
multiplication of effects.
Equally in the aggregate of individuals constituting a species, does
multiplication of effects become the continual cause of increasing
multiformity. The lapse of a species into divergent varieties, initiates
fresh combinations of forces tending to work further divergences. The new
varieties compete with the parent species in new ways; and so add new
elements to its circumstances. They modify somewhat the conditions of other
species existing in their habitat, or in the habitat they have invaded; and
the modifications wrought in such other species become additional sources
of influence. The Flora and Fauna of every region are united by their
entangled relations into a whole, of which no part can be affected without
affecting the rest. Hence, each differentiation in a local assemblage of
species, becomes the cause of further differentiations.
§ 156. One of the universal principles to which we saw that the
re-distribution of matter and motion conforms, is that in any aggregate
made up of mixed units, incident forces produce segregation--separate
unlike units and bring together like units; and it was shown that the
increasing integration and definiteness which characterizes each part of an
evolving organic aggregate, as of every other aggregate, results from this
(_First Principles_, § 166). It remains here to say that while the actions
and reactions between organisms and their changing environments, add to the
heterogeneity of organic structures, they also give to the heterogeneity
this growing distinctness. At first sight the reverse might be inferred. It
might be argued that any new set of effects wrought in an organism by some
new set of external forces, must tend more or less to obliterate the
effects previously wrought--must produce confusion or indefiniteness. A
little consideration, however, will dissipate this impression.
Doubtless the condition under which alone increasing definiteness of
structure can be acquired by any part of an organism, either in an
individual or in successive generations, is that such part shall be exposed
to some set of tolerably-constant forces; and doubtless, continual change
of circumstances interferes with this. But the interference can never be
considerable. For the pre-existing structure of an organism prevents it
from living under any new conditions except such as are congruous with the
fundamental characters of its organization--such as subject its essential
organs to actions substantially the same as before. Great changes must kill
it. Hence, it can continuously expose itself and its descendants, only to
those moderate changes which do not destroy the general harmony between the
aggregate of incident forces and the aggregate of its functions. That is,
it must remain under influences calculated to make greater the definiteness
of the chief differentiations already produced. If, for example, we set out
with an animal in which a rudimentary vertebral column with its attached
muscular system has been established; it is clear that the mechanical
arrangements have become thereby so far determined, that subsequent
modifications are extremely likely, if not certain, to be consistent with
the production of movement by the actions of muscles on a flexible central
axis. Hence, there will continue a general similarity in the play of forces
to which the flexible central axis is subject; and so, notwithstanding the
metamorphoses which the vertebrate type undergoes, there will be a
maintenance of conditions favourable to increasing definiteness and
integration of the vertebral column. Moreover, this maintenance of such
conditions becomes secure in proportion as organization advances. Each
further complexity of structure, implying some further complexity in the
relations between an organism and its environment, must tend to specialize
the actions and reactions between it and its environment--must tend to
increase the stringency with which it is restrained within such
environments as admit of those special actions and reactions for which its
structure fits it; that is, must further guarantee the continuance of those
actions and reactions to which its essential organs respond, and therefore
the continuance of the segregating process.
How in each species, considered as an aggregate of individuals, there must
arise stronger and stronger contrasts among those divergent varieties which
result from the instability of the homogeneous and the multiplication of
effects, need only be briefly indicated. It has already been shown (_First
Principles_, § 166), that in conformity to the universal law that mixed
units are segregated by like incident forces, there are produced
increasingly-definite distinctions among varieties, wherever there occur
definitely-distinguished sets of conditions to which the varieties are
respectively subject.
§ 157. Probably in the minds of some, the reading of this chapter has been
accompanied by a running commentary, to the effect that the argument proves
too much. The apparent implication is, that the passage from an indefinite,
incoherent homogeneity to a definite, coherent heterogeneity in organic
aggregates, must have been going on universally; whereas we find that in
many cases there has been persistence without progression. This apparent
implication, however, is not a real one.
For though every environment on the Earth's surface undergoes changes; and
though usually the organisms which each environment contains, cannot escape
certain resulting new influences; yet occasionally such new influences are
escaped, by the survival of species in the unchanged parts of their
habitats, or by their spread into neighbouring habitats which the change
has rendered like their original habitats, or by both. Any alteration in
the temperature of a climate or its degree of humidity, is unlikely to
affect simultaneously the whole area occupied by a species; and further, it
can scarcely fail to happen that the addition or subtraction of heat or
moisture, will give to a part of some adjacent area, a climate like that to
which the species has been habituated. If, again, the circumstances of a
species are modified by the intrusion of some foreign kind of plant or
animal, it follows that since the intruders will probably not spread
throughout its whole habitat, the species will, in one or more localities,
remain unaffected by them. Especially among marine creatures, must there
frequently occur cases in which modifying causes are continually eluded.
Comparatively uniform as are the physical conditions to which the sea
exposes its inhabitants, it becomes possible for such of them as live on
widely-diffused food, to be widely distributed; and wide distribution
generally prevents the members of a species from being all subject to the
same cause. Our commonest cirriped, for instance, subsisting on minute
creatures everywhere dispersed through the water; needing only to have some
firm surface on which to build up its shell; and in scarcely any danger
from surrounding animals; is able to exist on shores so widely remote from
one another, that nearly every change in the incident forces must fall
within narrower areas than that which the species occupies. Nearly always,
therefore, a portion of the species will survive unmodified. Its
easily-transported germs will take possession of such new habitats as have
been rendered fitter by the change that has unfitted some parts of its
original habitat. Hence, on successive occasions, while some parts of the
species are slightly transformed, another part may continually escape
transformation by migrating hither and thither, where the simple conditions
needed for its existence recur in nearly the same combinations as before.
And it will so become possible for it to survive, with insignificant
structural changes, throughout long geologic periods.
§ 158. The results to which we find ourselves led, are these.
In subordination to the different amounts and kinds of forces to which its
different parts are exposed, every individual organic aggregate, like all
other aggregates, tends to pass from its original indistinct simplicity
towards a more distinct complexity. Unless we deny the persistence of
force, we must admit that the lapse of an organism's structure from an
indefinitely homogeneous to a definitely heterogeneous state, must be
cumulative in successive generations, if the forces causing it continue to
act. And for the like reasons, the increasing assemblage of individuals
arising from a common stock, is also liable to lose its original
uniformity; and, in successive generations, to grow more pronounced in its
multiformity.
These changes, which would go to but a comparatively small extent were
organisms exposed to constant external conditions, are kept up by the
continual changes in external conditions, produced by astronomic, geologic,
meteorologic, and organic agencies: the average result being, that on
previous complications wrought by previous incident forces, new
complications are continually superposed by new incident forces. And hence
simultaneously arises increasing heterogeneity in the structures of
individuals, in the structures of species, and in the structures of the
Earth's Flora and Fauna.
But while, in very many or in most cases, the ever-changing incidence of
forces is ever adding to the complexity of organisms, and to the complexity
of the organic world as a whole; it does this only where its action cannot
be eluded. And since, by migration, it is possible for a species to keep
itself under conditions that are tolerably constant, there must be a
proportion of cases in which greater heterogeneity of structure is not to
be expected.
To show, however, that there must arise a certain average tendency to the
production of greater heterogeneity is not sufficient. Aggregates might be
rendered more heterogeneous by changing incident forces, without having
given to them that kind of heterogeneity required for carrying on life.
Hence it remains now to inquire how the production and maintenance of this
kind of heterogeneity is insured.