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.
If, then, there be truth in the assumption that the two elements, which, by
their union, give origin to a new organism, typify the two essential
constituents of such new organism, we must infer that the sperm-cell and
germ-cell respectively consist of co-ordinating matter and matter to be
co-ordinated--neurine and nutriment. That apparent identity of sperm-cell
and germ-cell seen in the lowest forms of life may thus be understood as
significant to the fact that no extended co-ordination of actions exists in
the generative product--each cell being a separate individual; and the
dissimilarity seen in higher organic types may, conversely, be understood
as expressive of, and consequent upon, the increasing degree of
co-ordination exhibited.[92]
That the sperm-cell and germ-cell are thus contrasted in nature and
function may further be suspected on considering the distinctive
characteristics of the sexes. Of the two elements they respectively
contribute to the formation of a fertile germ, it may be reasonably
supposed that each furnishes that which it possesses in greatest abundance
and can best spare. Well, in the greater size of the nervous centres in the
male, as well as in the fact that during famines men succumb sooner than
women, we see that in the male the co-ordinating system is relatively
predominant. From the same evidence, as well as from the greater abundance
of the cellular and adipose tissues in women, we may infer that the
nutritive system predominates in the female.[93] Here, then, is additional
support for the hypothesis that the sperm-cell, which is supplied by the
male, contains co-ordinating matter, and the germ-cell, which is supplied
by the female, contains matter to be co-ordinated.
The same inference may, again, be drawn from a general view of the maternal
function. For if, as we see, it is the office of the mother to afford milk
to the infant, and during a previous period to afford blood to the foetus,
it becomes probable that during a yet earlier stage it is still the
function to supply nutriment, though in another form. Indeed when,
ascending gradually the scale of animal life, we perceive that this
supplying of milk, and before that of blood, is simply a continuation of
the previous process, we may be sure that, with Nature's usual consistency,
this process is essentially one from the beginning.
Quite in harmony with this hypothesis concerning the respective natures of
the sperm-cell and germ-cell is a remark of Carpenter's on the same
point:--
"Looking," he says, "to the very equal mode in which the characters of
the two parents are mingled in _hybrid_ offspring, and to the certainty
that the _material_ conditions which determine the development of the
germ are almost exclusively female, it would seem probable that the
_dynamical_ conditions are, in great part, furnished by the male."[94]
§ 12. Could nothing but the foregoing indirect evidence be adduced in proof
of the proposition that the spermatozoon is essentially a neural element,
and the ovum essentially a hæmal element, we should scarcely claim for it
anything more than plausibility. On finding, however, that this indirect
evidence is merely introductory to evidence of a quite direct nature, its
significance will become apparent. Adding to their weight taken separately
the force of their mutual confirmation, these two series of proofs will be
seen to give the hypothesis a high degree of probability. The direct
evidence now to be considered is of several kinds.
On referring to the description of the process of multiplication in monads,
quoted some pages back (§ 5), from Professor Owen, the reader will perceive
that it is by the pellucid nucleus that the growth and reproduction of
these single-celled creatures are regulated. The nucleus controls the
circulation of the plasmatic fluid; the fission of the nucleus is the first
step towards the formation of another cell; each half of the divided
nucleus establishes round itself an independent current; and, apparently,
it is by the repulsion of the nuclei that the separation into two
individuals is finally effected. All which facts, when generalised, imply
that the nucleus is the governing or _co-ordinating_ part. Now, Professor
Owen subsequently points out that the matter of the sperm-cell performs in
the fertilised germ-cell just this same function which the nucleus performs
in a single-celled animal. We find the absorption by a germ-cell of the
contents of a sperm-cell "followed by the appearance of a pellucid nucleus
in the centre of the opaque and altered germ-cell; we further see its
successive fissions governed by the preliminary division of the pellucid
centre;" and, led by these and other facts, Professor Owen thinks that "one
cannot reasonably suppose that the nature and properties of the nucleus of
the impregnated germ-cell and that of the monad can be different."[95] And
hence he further infers that "the nucleus of the monad is of a nature
similar to, if not identical with," the matter of the spermatozoon. But we
have seen that in the monad the nucleus is the co-ordinating part; and
hence to say that the sperm-cell is, in nature, identical with it, is to
say that the sperm-cell consists of co-ordinating matter.
Chemical analysis affords further evidence, though, from the imperfect data
at present obtained, less conclusive evidence than could be wished. Partly
from the white and gray nervous substances having been analysed together
instead of separately, and partly from the difficulty of isolating the
efficient contents of the sperm-cells, a satisfactory comparison cannot be
made. Nevertheless, possessing in common, as they do, one element, by which
they are specially characterised, the analysis, as far as it goes, supports
our argument. The following table, which has been made up from data given
in the _Cyclopædia of Anatomy and Physiology, Art._ NERVOUS SYSTEM, gives
the proportion of this element in the brain in different conditions, and
shows how important is its presence.
+-----------------------------+--------+-------+-------+--------+-------+
| | In | In | In | In | In |
| |Infants.| Youth.|Adults.|Old Men.|Idiots.|
| +--------+-------+-------+--------+-------+
| Solid constituents in a | | | | | |
| hundred parts of the brain | 17.21 | 25.74 | 27.49 | 26.15 | 29.07 |
| Of these solid constituents | | | | | |
| the phosphorus amounts to | 0.8 | 1.65 | 1.80 | 1.00 | 0.85 |
| Which gives a percentage of | | | | | |
| phosphorus in the solid | | | | | |
| constituents of | 4.65 | 6.41 | 6.54 | 3.82 | 2.92 |
+-----------------------------+--------+-------+-------+--------+-------+
This connection between the quantity of phosphorus present and the degree
of mental power exhibited, is sufficiently significant; and the fact that
in the same individual the varying degrees of cerebral activity are
indicated by the varying quantities of alkaline phosphates excreted by the
kidneys,[96] still more clearly shows the essentialness of phosphorus as a
constituent of nervous matter. Respecting the constitution of sperm-cells
chemists do not altogether agree. One thing, however, is certain--that they
contain unoxidized phosphorus; and also a fatty acid, that is not
improbably similar to the fatty acid contained in neurine.[97] In fact,
there would seem to be present the constituents of that oleophosphoric acid
which forms so distinctive an element of the brain. That a large quantity
of binoxide of protein is also present, may be ascribed to the fact that a
great part of the sperm-cell consists merely of the protective membrane and
its locomotive appendage; the really efficient portion being but the
central contents.[98]
Evidence of a more conclusive nature--evidence, too, which will show in
what direction our argument tends--is seen in the marked antagonism of the
nervous and generative systems. Thus, the fact that intense mental
application, involving great waste of the nervous tissues, and a
corresponding consumption of nervous matter for their repair, is
accompanied by a cessation in the production of sperm-cells, gives strong
support to the hypothesis that the sperm-cells consist essentially of
neurine. And this becomes yet clearer on finding that the converse fact is
true--that undue production of sperm-cells involves cerebral inactivity.
The first result of a morbid excess in this direction is headache, which
may be taken to indicate that the brain is out of repair; this is followed
by stupidity; should the disorder continue, imbecility supervenes, ending
occasionally in insanity.
That the sperm-cell is co-ordinating matter, and the germ-cell matter to be
co-ordinated, is, therefore, an hypothesis not only having much _à priori_
probability, but one supported by numerous facts.
§ 13. This hypothesis alike explains, and is confirmed by, the truth, that
throughout the vertebrate tribes the degree of fertility varies inversely
as the development of the nervous system.
The necessary antagonism of Individuation and Reproduction does indeed show
itself amongst the higher animals, in some degree in the manner hitherto
traced; namely, as determining the total bulk. Though the parts now thrown
off, being no longer segments or gemmæ, are not obvious diminutions of the
parent, yet they must be really such. Under the form of internal fission,
the separative tendency is as much opposed to the aggregative tendency as
ever; and, _other things equal_, the greater or less development of the
individual depends upon the less or greater production of new individuals
or germs of new individuals. As in groups of cells, and series of groups of
cells, we saw that there was in each species a limit, passing which, the
germ product would not remain united; so in each species of higher animal
there is a limit, passing which, the process of cell-multiplication results
in the throwing off of cells, instead of resulting in the formation of more
tissue. Hence, taking an average view, we see why the smaller animals so
soon arrive at a reproductive age, and produce large and frequent broods;
and why, conversely, increased size is accompanied by retarded and
diminished fertility.
But, as above implied, it is not so much to the bulk of the body as a
whole, as to the bulk of the nervous system, that fertility stands related
amongst the higher animals. Probably, indeed, it stands thus related in all
cases; the difference simply arising from the fact, that whereas in the
lower organisms, where the nervous system is not concentrated, its bulk
varies as the bulk of the body, in the higher organisms it does not do so.
Be this as it may, however, we see clearly that, amongst the vertebrata,
the bodily development is not the determining circumstance. In a fish, a
reptile, a bird, and a mammal of the same weight, there is nothing like
equality of fecundity. Cattle and horses, arriving as they do so soon at a
reproductive age, are much more prolific than the human race, at the same
time that they are much larger. And whilst, again, the difference in size
between the elephant and man is far greater, their respective powers of
multiplication are less unlike. Looking in these cases at the nervous
systems, however, we find no such discrepancy. On learning that the average
ratio of the brain to the body is--in fishes, 1 to 5668; in reptiles, 1 to
1321; in birds, 1 to 212; and in mammals, 1 to 186;[99] their different
degrees of fecundity are accounted for. Though an ox will outweigh
half-a-dozen men, yet its brain and spinal cord are far less than those of
one man; and though in bodily development the elephant so immensely exceeds
the human being, yet the elephant's cerebro-spinal system is only thrice
the size attained by that of civilized men.[100] Unfortunately, it is
impossible to trace throughout the animal kingdom this inverse relationship
between the nervous and reproductive systems with any accuracy. Partly from
the fact that, in each case, the degree of fertility depends on three
variable elements--the age at which reproduction begins, the number
produced at a birth, and the frequency of the births; partly from the fact
that, in respect to most animals, these data are not satisfactorily
attainable, and that, when they are attainable, they are vitiated by the
influence of domesticity; and partly from the fact that no precise
measurement of the respective nervous systems has been made, we are unable
to draw any but general and somewhat vague comparisons. These, however, as
far as they go, are in our favour. Ascending from beings of the acrite
nerveless type, which are the most prolific of all, through the various
invertebrate sub-kingdoms, amongst which spontaneous fission disappears as
the nervous system becomes developed; passing again to the least nervous
and most fertile of the vertebrate series--Fishes, of which, too, the
comparatively large-brained cartilaginous kinds multiply much less rapidly
than the others; progressing through the more highly endowed and less
prolific Reptiles to the Mammalia, amongst which the Rodents, with their
unconvoluted brains, are noted for their fecundity; and ending with man and
the elephant, the least fertile and largest-brained of all--there seems to
be throughout a constant relationship between these attributes.
And indeed, on turning back to our _à priori_ principle, no other
relationship appears possible. We found it to be the necessary law of
maintenance of races, that the ability to maintain individual life and the
ability to multiply vary inversely. But the ability to maintain individual
life _is in all cases measured by the development of the nervous system_.
If it be in good visceral organization that the power of self-preservation
is shown, this implies some corresponding nervous apparatus to secure
sufficient food. If it be in strength, there must be a provision of nerves
and nervous centres answering to the number and size of the muscles. If it
be in swiftness and agility, a proportionate development of the cerebellum
is presupposed. If it be in intelligence, this varies with the size of the
cerebrum. As in all cases co-ordination of actions constitutes the life,
or, what is the same thing, the ability to maintain life; and as throughout
the animal kingdom this co-ordination, under all its forms, is effected by
nervous agents of some kind or other; and as each of these nervous agents
performs but one function; it follows that in proportion to the number of
the actions co-ordinated must be the number of nervous agents. Hence the
nervous system becomes the universal measure of the degree of co-ordination
of actions; that is, of the life, or ability to maintain life. And if the
nervous system varies directly as the ability to maintain life, it _must_
vary inversely as the ability to multiply.[101]
And here, assuming the constitution of the sperm-cell above inferred to be
the true one, we see how the obverse _à priori_ principle is fulfilled.
Where, as amongst the lowest organisms, bulk is expressive of life, the
antagonism of individuation and reproduction was broadly exhibited in the
fact that the making of two or more new individuals was the _un_making of
the original individual. And now, amongst the higher organisms, where bulk
is no longer the measure of life, we see that this antagonism is between
the neural elements thrown off, and that internal neural mass whose bulk
_is_ the measure of life. The production of co-ordinating cells must be at
the expense of the co-ordinating apparatus; and the aggregation of the
co-ordinating apparatus must be at the expense of co-ordinating cells. How
the antagonism affects the female economy is not so clear. Possibly the
provision required to be made for supplying nervous as well as other
nutriment to the embryo, involves an arrest in the development of the
nervous system; and if so, probably this arrest takes place early in
proportion as the number of the coming offspring makes the required
provision great: or rather, to put the facts in their right sequence, an
early arrest renders the production of a numerous offspring possible.
§ 14. The law which we have thus traced throughout the animal kingdom, and
which must alike determine the different fertilities of different species,
and the variations of fertility in the same species, we have now to
consider in its application to mankind.
[_The remainder of the essay, which as implied, deals with the
application of this general principle to the multiplication of the human
race, need not be here reproduced. The subject is treated in full in Part
VI._]
APPENDIX B.
THE INADEQUACY OF NATURAL SELECTION, ETC., ETC.
[_In this Appendix are included four essays originally published in the_
Contemporary Review _and subsequently republished as pamphlets. The first
appeared under the above title in February and March, 1893; the second in
May of that year under the title "Prof. Weismann's Theories;" the third in
December of that year under the title "A Rejoinder to Prof. Weismann;" and
the fourth in October, 1894, under the title "Weismannism Once More." As
these successive essays practically form parts of one whole, I have thought
it needless to keep them separate by repeating their titles, and have
simply marked them off from one another by the numbers I, II, III, IV. Of
course, as they are components of a controversy, some incompleteness arises
from the absence of the essays to which portions of them were replies; but
in each the course of the argument sufficiently indicates the
counter-arguments which were met._]
I.
Students of psychology are familiar with the experiments of Weber on the
sense of touch. He found that different parts of the surface differ widely
in their ability to give information concerning the things touched. Some
parts, which yielded vivid sensations, yielded little or no knowledge of
the sizes or forms of the things exciting them; whereas other parts, from
which there came sensations much less acute, furnished clear impressions
respecting the tangible characters, even of relatively small objects. These
unlikenesses of tactual discriminativeness he ingeniously expressed by
actual measurements. Taking a pair of compasses, he found that if they were
closed so nearly that the points were less than one-twelfth of an inch
apart, the end of the forefinger could not perceive that there were two
points: the two points seemed one. But when the compasses were opened so
that the points were one-twelfth of an inch apart, then the end of the
forefinger distinguished the two points. At the same time, he found that
the compasses must be opened to the extent of two and a half inches, before
the middle of the back could distinguish between two points and one. That
is to say, as thus measured, the end of the forefinger has thirty times the
tactual discriminativeness which the middle of the back has.
Between these extremes he found gradations. The inner surfaces of the
second joints of the fingers can distinguish separateness of positions only
half as well as the tip of the forefinger. The innermost joints are still
less discriminating, but have powers of discrimination equal to that of the
tip of the nose. The end of the great toe, the palm of the hand, and the
cheek, have alike one-fifth of the perceptiveness which the tip of the
forefinger has; and the lower part of the forehead has but one-half that
possessed by the cheek. The back of the hand and the crown of the head are
nearly alike in having but a fourteenth or a fifteenth of the ability to
perceive positions as distinct, which is possessed by the finger-end. The
thigh, near the knee, has rather less, and the breast less still; so that
the compasses must be opened more than an inch and a half before the breast
distinguishes the two points from one another.
What is the meaning of these differences? How, in the course of evolution,
have they been established? If "natural selection," or survival of the
fittest, is the assigned cause, then it is required to show in what way
each of these degrees of endowment has advantaged the possessor to such
extent that not infrequently life has been directly or indirectly preserved
by it. We might reasonably assume that in the absence of some
differentiating process, all parts of the surface would have like powers of
perceiving relative positions. They cannot have become widely unlike in
perceptiveness without some cause. And if the cause alleged is natural
selection, then it is necessary to show that the greater degree of the
power possessed by this part than by that, has not only conduced to the
maintenance of life, but has conduced so much that an individual in whom a
variation has produced better adjustment to needs, thereby maintained life
when some others lost it; and that among the descendants inheriting this
variation, there was a derived advantage such as enabled them to multiply
more than the descendants of individuals not possessing it. Can this, or
anything like this, be shown?
That the superior perceptiveness of the forefinger-tip has thus arisen,
might be contended with some apparent reason. Such perceptiveness is an
important aid to manipulation, and may have sometimes given a life-saving
advantage. In making arrows or fish-hooks, a savage possessing some extra
amount of it may have been thereby enabled to get food where another
failed. In civilized life, too, a sempstress with well-endowed finger-ends
might be expected to gain a better livelihood than one with finger-ends
which were obtuse; though this advantage would not be so great as appears.
I have found that two ladies whose finger-ends were covered with
glove-tips, reducing their sensitiveness from one-twelfth of an inch
between compass-points to one-seventh, lost nothing appreciable of their
quickness and goodness in sewing. An experience of my own here comes in
evidence. Towards the close of my salmon-fishing days I used to observe
what a bungler I had become in putting on and taking off artificial flies.
As the tactual discriminativeness of my finger-ends, recently tested, comes
up to the standard specified by Weber, it is clear that this decrease of
manipulative power, accompanying increase of age, was due to decrease in
the delicacy of muscular co-ordination and sense of pressure--not to
decrease of tactual discriminativeness. But not making much of these
criticisms, let us admit the conclusion that this high perceptive power
possessed by the forefinger-end may have arisen by survival of the fittest;
and let us limit the argument to the other differences.
How about the back of the trunk and its face? Is any advantage derived from
possession of greater tactual discriminativeness by the last than the
first? The tip of the nose has more than three times the power of
distinguishing relative positions which the lower part of the forehead has.
Can this greater power be shown to have any advantage? The back of the hand
has scarcely more discriminative ability than the crown of the head, and
has only one-fourteenth of that which the finger-tip has. Why is this?
Advantage might occasionally be derived if the back of the hand could tell
us more than it does about the shapes of the surfaces touched. Why should
the thigh near the knee be twice as perceptive as the middle of the thigh?
And, last of all, why should the middle of the forearm, middle of the
thigh, middle of the back of the neck, and middle of the back, all stand on
the lowest level, as having but one-thirtieth of the perceptive power which
the tip of the forefinger has? To prove that these differences have arisen
by natural selection, it has to be shown that such small variation in one
of the parts as might occur in a generation--say one-tenth extra
amount--has yielded an appreciably greater power of self-preservation; and
that those inheriting it have continued to be so far advantaged as to
multiply more than those who, in other respects equal, were less endowed
with this trait. Does any one think he can show this?
But if this distribution of tactual perceptiveness cannot be explained by
survival of the fittest, how can it be explained? The reply is that, if
there has been in operation a cause which it is now the fashion among
biologists to ignore or deny, these various differences are at once
accounted for. This cause is the inheritance of acquired characters. As a
preliminary to setting forth the argument showing this, I have made some
experiments.
It is a current belief that the fingers of the blind, more practised in
tactual exploration than the fingers of those who can see, acquire greater
discriminativeness: especially the fingers of those blind who have been
taught to read from raised letters. Not wishing to trust to this current
belief, I recently tested two youths, one of fifteen and the other younger,
at the School for the Blind in Upper Avenue Road, and found the belief to
be correct. I found that instead of being unable to distinguish between
points of the compasses until they were opened to one-twelfth of an inch
apart, both of them could distinguish between points when only
one-fourteenth of an inch apart. They had thick and coarse skins; and
doubtless, had the intervening obstacle, so produced, been less, the
discriminative power would have been greater. It afterwards occurred to me
that a better test would be furnished by those whose finger-ends are
exercised in tactual perceptions, not occasionally, as by the blind in
reading, but all day long in pursuit of their occupations. The facts
answered expectation. Two skilled compositors, on whom I experimented, were
both able to distinguish between points when they were only one-seventeenth
of an inch apart. Thus we have clear proof that constant exercise of the
tactual nervous structure leads to further development.[102]
Now if acquired structural traits are inheritable, the various contrasts
above set down are obvious consequences; for the gradations in tactual
perceptiveness correspond with the gradations in the tactual exercises of
the parts. Save by contact with clothes, which present only broad surfaces
having but slight and indefinite contrast, the trunk has scarcely any
converse with external bodies, and it has but small discriminative power;
but what discriminative power it has is greater on its face than on its
back, corresponding to the fact that the chest and abdomen are much more
frequently explored by the hands: this difference being probably in part
inherited from inferior creatures; for, as we may see in dogs and cats, the
belly is far more accessible to feet and tongue than the back. No less
obtuse than the back are the middle of the back of the neck, the middle of
the forearm, and the middle of the thigh; and these parts have but rare
experiences of irregular foreign bodies. The crown of the head is
occasionally felt by the fingers, as also the back of one hand by the
fingers of the other; but neither of these surfaces, which are only twice
as perceptive as the back, is used with any frequency for touching objects,
much less for examining them. The lower part of the forehead, though more
perceptive than the crown of the head, in correspondence with a somewhat
greater converse with the hands, is less than one-third as perceptive as
the tip of the nose; and manifestly, both in virtue of its relative
prominence, in virtue of its contacts with things smelt at, and in virtue
of its frequent acquaintance with the handkerchief, the tip of the nose has
far greater tactual experience. Passing to the inner surfaces of the hands,
which, taken as wholes, are more constantly occupied in touching than are
the back, breast, thigh, forearm, forehead, or back of the hand, Weber's
scale shows that they are much more perceptive, and that the degrees of
perceptiveness of different parts correspond with their tactual activities.
The palms have but one-fifth the perceptiveness possessed by the
forefinger-ends; the inner surfaces of the finger-joints next the palms
have but one-third; while the inner surfaces of the second joints have but
one-half. These abilities correspond with the facts that whereas the inner
parts of the hand are used only in grasping things, the tips of the fingers
come into play not only when things are grasped, but when such things, as
well as smaller things, are felt at or manipulated. It needs but to observe
the relative actions of these parts in writing, in sewing, in judging
textures, &c., to see that above all other parts the finger-ends, and
especially the forefinger-ends, have the most multiplied experiences. If,
then, it be that the extra perceptiveness acquired from actual tactual
activities, as in a compositor, is inheritable, these gradations of tactual
perceptiveness are explained.
Doubtless some of those who remember Weber's results, have had on the tip
of the tongue the argument derived from the tip of the tongue. This part
exceeds all other parts in power of tactual discrimination: doubling, in
that respect, the power of the forefinger-tip. It can distinguish points
that are only one-twenty-fourth of an inch apart. Why this unparalleled
perceptiveness? If survival of the fittest be the ascribed cause, then it
has to be shown what the advantages achieved have been; and, further, that
those advantages have been sufficiently great to have had effects on the
maintenance of life.