THE FACTORS
OF
ORGANIC EVOLUTION.
BY
HERBERT SPENCER.
REPRINTED, WITH ADDITIONS, FROM
_THE NINETEENTH CENTURY_.
NEW YORK:
D. APPLETON AND COMPANY,
1, 8, AND 5 BOND STREET.
1887.
PREFACE.
The two parts of which this Essay consists, originally published in _The
Nineteenth Century_ for April and May 1886 respectively, now reappear
with the assent of the proprietor and editor of that periodical, to whom
my thanks are due for his courtesy in giving it. Some passages of
considerable length which, with a view to needful brevity, were omitted
when the articles first appeared, have been restored.
Though the direct bearings of the arguments contained in this Essay are
biological, the argument contained in its first half has indirect
bearings upon Psychology, Ethics, and Sociology. My belief in the
profound importance of these indirect bearings, was originally a chief
prompter to set forth the argument; and it now prompts me to re-issue it
in permanent form.
Though mental phenomena of many kinds, and especially of the simpler
kinds, are explicable only as resulting from the natural selection of
favourable variations; yet there are, I believe, still more numerous
mental phenomena, including all those of any considerable complexity,
which cannot be explained otherwise than as results of the inheritance
of functionally-produced modifications. What theory of psychological
evolution is espoused, thus depends on acceptance or rejection of the
doctrine that not only in the individual, but in the successions of
individuals, use and disuse of parts produce respectively increase and
decrease of them.
Of course there are involved the conceptions we form of the genesis and
nature of our higher emotions; and, by implication, the conceptions we
form of our moral intuitions. If functionally-produced modifications are
inheritable, then the mental associations habitually produced in
individuals by experiences of the relations between actions and their
consequences, pleasurable or painful, may, in the successions of
individuals, generate innate tendencies to like or dislike such actions.
But if not, the genesis of such tendencies is, as we shall see, not
satisfactorily explicable.
That our sociological beliefs must also be profoundly affected by the
conclusions we draw on this point, is obvious. If a nation is modified
_en masse_ by transmission of the effects produced on the natures of its
members by those modes of daily activity which its institutions and
circumstances involve; then we must infer that such institutions and
circumstances mould its members far more rapidly and comprehensively
than they can do if the sole cause of adaptation to them is the more
frequent survival of individuals who happen to have varied in favourable
ways.
I will add only that, considering the width and depth of the effects
which acceptance of one or other of these hypotheses must have on our
views of Life, Mind, Morals, and Politics, the question—Which of them is
true? demands, beyond all other questions whatever, the attention of
scientific men.
_Brighton, January, 1887._
THE FACTORS OF ORGANIC EVOLUTION.
[_April and May, 1886_]
I.
Within the recollection of men now in middle life, opinion concerning
the derivation of animals and plants was in a chaotic state. Among the
unthinking there was tacit belief in creation by miracle, which formed
an essential part of the creed of Christendom; and among the thinking
there were two parties, each of which held an indefensible hypothesis.
Immensely the larger of these parties, including nearly all whose
scientific culture gave weight to their judgments, though not accepting
literally the theologically-orthodox doctrine, made a compromise between
that doctrine and the doctrines which geologists had established; while
opposed to them were some, mostly having no authority in science, who
held a doctrine which was heterodox both theologically and
scientifically. Professor Huxley, in his lecture on “The Coming of Age
of the Origin of Species,” remarks concerning the first of these parties
as follows:—
“One-and-twenty years ago, in spite of the work commenced by Hutton
and continued with rare skill and patience by Lyell, the dominant
view of the past history of the earth was catastrophic. Great and
sudden physical revolutions, wholesale creations and extinctions of
living beings, were the ordinary machinery of the geological epic
brought into fashion by the misapplied genius of Cuvier. It was
gravely maintained and taught that the end of every geological epoch
was signalised by a cataclysm, by which every living being on the
globe was swept away, to be replaced by a brand-new creation when
the world returned to quiescence. A scheme of nature which appeared
to be modelled on the likeness of a succession of rubbers of whist,
at the end of each of which the players upset the table and called
for a new pack, did not seem to shock anybody.
I may be wrong, but I doubt if, at the present time, there is a
single responsible representative of these opinions left. The
progress of scientific geology has elevated the fundamental
principle of uniformitarianism, that the explanation of the past is
to be sought in the study of the present, into the position of an
axiom; and the wild speculations of the catastrophists, to which we
all listened with respect a quarter of a century ago, would hardly
find a single patient hearer at the present day.”
Of the party above referred to as not satisfied with this conception
described by Professor Huxley, there were two classes. The great
majority were admirers of the _Vestiges of the Natural History of
Creation_—a work which, while it sought to show that organic evolution
has taken place, contended that the cause of organic evolution, is “an
impulse” supernaturally “imparted to the forms of life, advancing them,
... through grades of organization.” Being nearly all very inadequately
acquainted with the facts, those who accepted the view set forth in the
_Vestiges_ were ridiculed by the well-instructed for being satisfied
with evidence, much of which was either invalid or easily cancelled by
counter-evidence, and at the same time they exposed themselves to the
ridicule of the more philosophical for being content with a supposed
explanation which was in reality no explanation: the alleged “impulse”
to advance giving us no more help in understanding the facts than does
Nature's alleged “abhorrence of a vacuum” help us to understand the
ascent of water in a pump. The remnant, forming the second of these
classes, was very small. While rejecting this mere verbal solution,
which both Dr. Erasmus Darwin and Lamarck had shadowed forth in other
language, there were some few who, rejecting also the hypothesis
indicated by both Dr. Darwin and Lamarck, that the promptings of desires
or wants produced growths of the parts subserving them, accepted the
single _vera causa_ assigned by these writers—the modification of
structures resulting from modification of functions. They recognized as
the sole process in organic development, the adaptation of parts and
powers consequent on the effects of use and disuse—that continual
moulding and re-moulding of organisms to suit their circumstances, which
is brought about by direct converse with such circumstances.
But while this cause accepted by these few is a true cause, since
unquestionably during the life of the individual organism changes of
function produce changes of structure; and while it is a tenable
hypothesis that changes of structure so produced are inheritable; yet it
was manifest to those not prepossessed, that this cause cannot with
reason be assigned for the greater part of the facts. Though in plants
there are some characters which may not irrationally be ascribed to the
direct effects of modified functions consequent on modified
circumstances, yet the majority of the traits presented by plants are
not to be thus explained. It is impossible that the thorns by which a
briar is in large measure defended against browsing animals, can have
been developed and moulded by the continuous exercise of their
protective actions; for in the first place, the great majority of the
thorns are never touched at all, and, in the second place, we have no
ground whatever for supposing that those which are touched are thereby
made to grow, and to take those shapes which render them efficient.
Plants which are rendered uneatable by the thick woolly coatings of
their leaves, cannot have had these coatings produced by any process of
reaction against the action of enemies; for there is no imaginable
reason why, if one part of a plant is eaten, the rest should thereafter
begin to develop the hairs on its surface. By what direct effect of
function on structure, can the shell of a nut have been evolved? Or how
can those seeds which contain essential oils, rendering them unpalatable
to birds, have been made to secrete such essential oils by these actions
of birds which they restrain? Or how can the delicate plumes borne by
some seeds, and giving the wind power to waft them to new stations, be
due to any immediate influences of surrounding conditions? Clearly in
these and in countless other cases, change of structure cannot have been
directly caused by change of function. So is it with animals to a large
extent, if not to the same extent. Though we have proof that by rough
usage the dermal layer may be so excited as to produce a greatly
thickened epidermal layer, sometimes quite horny; and though it is a
feasible hypothesis that an effect of this kind persistently produced
may be inherited; yet no such cause can explain the carapace of the
turtle, the armour of the armadillo, or the imbricated covering of the
manis. The skins of these animals are no more exposed to habitual hard
usage than are those of animals covered by hair. The strange
excrescences which distinguish the heads of the hornbills, cannot
possibly have arisen from any reaction against the action of surrounding
forces; for even were they clearly protective, there is no reason to
suppose that the heads of these birds need protection more than the
heads of other birds. If, led by the evidence that in animals the amount
of covering is in some cases affected by the degree of exposure, it were
admitted as imaginable that the development of feathers from preceding
dermal growths had resulted from that extra nutrition caused by extra
superficial circulation, we should still be without explanation of the
structure of a feather. Nor should we have any clue to the specialities
of feathers—the crests of various birds, the tails sometimes so
enormous, the curiously placed plumes of the bird of paradise, &c., &c.
Still more obviously impossible is it to explain as due to use or disuse
the colours of animals. No direct adaptation to function could have
produced the blue protuberances on a mandril's face, or the striped hide
of a tiger, or the gorgeous plumage of a kingfisher, or the eyes in a
peacock's tail, or the multitudinous patterns of insects' wings. One
single case, that of a deer's horns, might alone have sufficed to show
how insufficient was the assigned cause. During their growth, a deer's
horns are not used at all; and when, having been cleared of the dead
skin and dried-up blood-vessels covering them, they are ready for use,
they are nerveless and non-vascular, and hence are incapable of
undergoing any changes of structure consequent on changes of function.
Of these few then, who rejected the belief described by Professor
Huxley, and who, espousing the belief in a continuous evolution, had to
account for this evolution, it must be said that though the cause
assigned was a true cause, yet, even admitting that it operated through
successive generations, it left unexplained the greater part of the
facts. Having been myself one of these few, I look back with surprise at
the way in which the facts which were congruous with the espoused view
monopolized consciousness and kept out the facts which were incongruous
with it—conspicuous though many of them were. The misjudgment was not
unnatural. Finding it impossible to accept any doctrine which implied a
breach in the uniform course of natural causation, and, by implication,
accepting as unquestionable the origin and development of all organic
forms by accumulated modifications naturally caused, that which appeared
to explain certain classes of these modifications, was supposed to be
capable of explaining the rest: the tendency being to assume that these
would eventually be similarly accounted for, though it was not clear
how.
Returning from this parenthetic remark, we are concerned here chiefly to
remember that, as said at the outset, there existed thirty years ago, no
tenable theory about the genesis of living things. Of the two
alternative beliefs, neither would bear critical examination.
* * * * *
Out of this dead lock we were released—in large measure, though not I
believe entirely—by the _Origin of Species_. That work brought into view
a further factor; or rather, such factor, recognized as in operation by
here and there an observer (as pointed out by Mr. Darwin in his
introduction to the second edition), was by him for the first time seen
to have played so immense a part in the genesis of plants and animals.
Though laying myself open to the charge of telling a thrice-told tale, I
feel obliged here to indicate briefly the several great classes of facts
which Mr. Darwin's hypothesis explains; because otherwise that which
follows would scarcely be understood. And I feel the less hesitation in
doing this because the hypothesis which it replaced, not very widely
known at any time, has of late so completely dropped into the
background, that the majority of readers are scarcely aware of its
existence, and do not therefore understand the relation between Mr.
Darwin's successful interpretation and the preceding unsuccessful
attempt at interpretation. Of these classes of facts, four chief ones
may be here distinguished.
In the first place, such adjustments as those exemplified above are made
comprehensible. Though it is inconceivable that a structure like that of
the pitcher-plant could have been produced by accumulated effects of
function on structure; yet it is conceivable that successive selections
of favourable variations might have produced it; and the like holds of
the no less remarkable appliance of the Venus's Fly-trap, or the still
more astonishing one of that water-plant by which infant-fish are
captured. Though it is impossible to imagine how, by direct influence of
increased use, such dermal appendages as a porcupine's quills could have
been developed; yet, profiting as the members of a species otherwise
defenceless might do by the stiffness of their hairs, rendering them
unpleasant morsels to eat, it is a feasible supposition that from
successive survivals of individuals thus defended in the greatest
degrees, and the consequent growth in successive generations of hairs
into bristles, bristles into spines, spines into quills (for all these
are homologous), this change could have arisen. In like manner, the odd
inflatable bag of the bladder-nosed seal, the curious fishing-rod with
its worm-like appendage carried on the head of the _lophius_ or angler,
the spurs on the wings of certain birds, the weapons of the sword-fish
and saw-fish, the wattles of fowls, and numberless such peculiar
structures, though by no possibility explicable as due to effects of use
or disuse, are explicable as resulting from natural selection operating
in one or other way.
In the second place, while showing us how there have arisen countless
modifications in the forms, structures, and colours of each part, Mr.
Darwin has shown us how, by the establishment of favourable variations,
there may arise new parts. Though the first step in the production of
horns on the heads of various herbivorous animals, may have been the
growth of callosities consequent on the habit of butting—such
callosities thus functionally initiated being afterwards developed in
the most advantageous ways by selection; yet no explanation can be thus
given of the sudden appearance of a duplicate set of horns, as
occasionally happens in sheep: an addition which, where it proved
beneficial, might readily be made a permanent trait by natural
selection. Again, the modifications which follow use and disuse can by
no possibility account for changes in the numbers of vertebræ; but after
recognizing spontaneous, or rather fortuitous, variation as a factor, we
can see that where an additional vertebra hence resulting (as in some
pigeons) proves beneficial, survival of the fittest may make it a
constant character; and there may, by further like additions, be
produced extremely long strings of vertebræ, such as snakes show us.
Similarly with the mammary glands. It is not an unreasonable supposition
that by the effects of greater or less function, inherited through
successive generations, these may be enlarged or diminished in size; but
it is out of the question to allege such a cause for changes in their
numbers. There is no imaginable explanation of these save the
establishment by inheritance of spontaneous variations, such as are
known to occur in the human race.
So too, in the third place, with certain alterations in the connections
of parts. According to the greater or smaller demands made on this or
that limb, the muscles moving it may be augmented or diminished in bulk;
and, if there is inheritance of changes so wrought, the limb may, in
course of generations, be rendered larger or smaller. But changes in the
arrangements or attachments of muscles cannot be thus accounted for. It
is found, especially at the extremities, that the relations of tendons
to bones and to one another are not always the same. Variations in their
modes of connection may occasionally prove advantageous, and may thus
become established. Here again, then, we have a class of structural
changes to which Mr. Darwin's hypothesis gives us the key, and to which
there is no other key.
Once more there are the phenomena of mimicry. Perhaps in a more striking
way than any others, these show how traits which seem inexplicable are
explicable as due to the more frequent survival of individuals that have
varied in favorable ways. We are enabled to understand such marvelous
simulations as those of the leaf-insect, those of beetles which
“resemble glittering dew-drops upon the leaves;” those of caterpillars
which, when asleep, stretch themselves out so as to look like twigs. And
we are shown how there have arisen still more astonishing
imitations—those of one insect by another. As Mr. Bates has proved,
there are cases in which a species of butterfly, rendered so unpalatable
to insectivorous birds by its disagreeable taste that they will not
catch it, is simulated in its colors and markings by a species which is
structurally quite different—so simulated that even a practiced
entomologist is liable to be deceived: the explanation being that an
original slight resemblance, leading to occasional mistakes on the part
of birds, was increased generation after generation by the more frequent
escape of the most-like individuals, until the likeness became thus
great.
But now, recognizing in full this process brought into clear view by Mr.
Darwin, and traced out by him with so much care and skill, can we
conclude that, taken alone, it accounts for organic evolution? Has the
natural selection of favourable variations been the sole factor? On
critically examining the evidence, we shall find reason to think that it
by no means explains all that has to be explained. Omitting for the
present any consideration of a factor which may be distinguished as
primordial, it may be contended that the above-named factor alleged by
Dr. Erasmus Darwin and by Lamarck, must be recognized as a co-operator.
Utterly inadequate to explain the major part of the facts as is the
hypothesis of the inheritance of functionally-produced modifications,
yet there is a minor part of the facts, very extensive though less,
which must be ascribed to this cause.
* * * * *
When discussing the question more than twenty years ago (_Principles of
Biology_, § 166), I instanced the decreased size of the jaws in the
civilized races of mankind, as a change not accounted for by the natural
selection of favourable variations; since no one of the decrements by
which, in thousands of years, this reduction has been effected, could
have given to an individual in which it occurred, such advantage as
would cause his survival, either through diminished cost of local
nutrition or diminished weight to be carried. I did not then exclude, as
I might have done, two other imaginable causes. It may be said that
there is some organic correlation between increased size of brain and
decreased size of jaw: Camper's doctrine of the facial angle being
referred to in proof. But this argument may be met by pointing to the
many examples of small-jawed people who are also small-brained, and by
citing not infrequent cases of individuals remarkable for their mental
powers, and at the same time distinguished by jaws not less than the
average but greater. Again, if sexual selection be named as a possible
cause, there is the reply that, even supposing such slight diminution of
jaw as took place in a single generation to have been an attraction, yet
the other incentives to choice on the part of men have been too many and
great to allow this one to weigh in an adequate degree; while, during
the greater portion of the period, choice on the part of women has
scarcely operated: in earlier times they were stolen or bought, and in
later times mostly coerced by parents. Thus, reconsideration of the
facts does not show me the invalidity of the conclusion drawn, that this
decrease in size of jaw can have had no other cause than continued
inheritance of those diminutions consequent on diminutions of function,
implied by the use of selected and well-prepared food. Here, however, my
chief purpose is to add an instance showing, even more clearly, the
connexion between change of function and change of structure. This
instance, allied in nature to the other, is presented by those
varieties, or rather sub-varieties, of dogs, which, having been
household pets, and habitually fed on soft food, have not been called on
to use their jaws in tearing and crunching, and have been but rarely
allowed to use them in catching prey and in fighting. No inference can
be drawn from the sizes of the jaws themselves, which, in these dogs,
have probably been shortened mainly by selection. To get direct proof of
the decrease of the muscles concerned in closing the jaws or biting,
would require a series of observations very difficult to make. But it is
not difficult to get indirect proof of this decrease by looking at the
bony structures with which these muscles are connected. Examination of
the skulls of sundry indoor dogs contained in the Museum of the College
of Surgeons, proves the relative smallness of such parts. The only
pug-dog's skull is that of an individual not perfectly adult; and though
its traits are quite to the point they cannot with safety be taken as
evidence. The skull of a toy-terrier has much restricted areas of
insertion for the temporal muscles; has weak zygomatic arches; and has
extremely small attachments for the masseter muscles. Still more
significant is the evidence furnished by the skull of a King Charles's
spaniel, which, if we allow three years to a generation, and bear in
mind that the variety must have existed before Charles the Second's
reign, we may assume belongs to something approaching to the hundredth
generation of these household pets. The relative breadth between the
outer surfaces of the zygomatic arches is conspicuously small; the
narrowness of the temporal fossæ is also striking; the zygomata are very
slender; the temporal muscles have left no marks whatever, either by
limiting lines or by the character of the surfaces covered; and the
places of attachment for the masseter muscles are very feebly developed.
At the Museum of Natural History, among skulls of dogs there is one
which, though unnamed, is shown by its small size and by its teeth, to
have belonged to one variety or other of lap-dogs, and which has the
same traits in an equal degree with the skull just described. Here,
then, we have two if not three kinds of dogs which, similarly leading
protected and pampered lives, show that in the course of generations the
parts concerned in clenching the jaws have dwindled. To what cause must
this decrease be ascribed? Certainly not to artificial selection; for
most of the modifications named make no appreciable external signs: the
width across the zygomata could alone be perceived. Neither can natural
selection have had anything to do with it; for even were there any
struggle for existence among such dogs, it cannot be contended that any
advantage in the struggle could be gained by an individual in which a
decrease took place. Economy of nutrition, too, is excluded. Abundantly
fed as such dogs are, the constitutional tendency is to find places
where excess of absorbed nutriment may be conveniently deposited, rather
than to find places where some cutting down of the supplies is
practicable. Nor again can there be alleged a possible correlation
between these diminutions and that shortening of the jaws which has
probably resulted from selection; for in the bull-dog, which has also
relatively short jaws, these structures concerned in closing them are
unusually large. Thus there remains as the only conceivable cause, the
diminution of size which results from diminished use. The dwindling of a
little-exercised part has, by inheritance, been made more and more
marked in successive generations.
* * * * *
Difficulties of another class may next be exemplified—those which
present themselves when we ask how there can be effected by the
selection of favourable variations, such changes of structure as adapt
an organism to some useful action in which many different parts
co-operate. None can fail to see how a simple part may, in course of
generations, be greatly enlarged, if each enlargement furthers, in some
decided way, maintenance of the species. It is easy to understand, too,
how a complex part, as an entire limb, may be increased as a whole by
the simultaneous due increase of its co-operative parts; since if, while
it is growing, the channels of supply bring to the limb an unusual
quantity of blood, there will naturally result a proportionately greater
size of all its components—bones, muscles, arteries, veins, &c. But
though in cases like this, the co-operative parts forming some large
complex part may be expected to vary together, nothing implies that they
necessarily do so; and we have proof that in various cases, even when
closely united, they do not do so. An example is furnished by those
blind crabs named in the _Origin of Species_ which inhabit certain dark
caves of Kentucky, and which, though they have lost their eyes, have not
lost the foot-stalks which carried their eyes. In describing the
varieties which have been produced by pigeon-fanciers, Mr. Darwin notes
the fact that along with changes in length of beak produced by
selection, there have not gone proportionate changes in length of
tongue. Take again the case of teeth and jaws. In mankind these have not
varied together. During civilization the jaws have decreased, but the
teeth have not decreased in proportion; and hence that prevalent
crowding of them, often remedied in childhood by extraction of some, and
in other cases causing that imperfect development which is followed by
early decay. But the absence of proportionate variation in co-operative
parts that are close together, and are even bound up in the same mass,
is best seen in those varieties of dogs named above as illustrating the
inherited effects of disuse. We see in them, as we see in the human
race, that diminution in the jaws has not been accompanied by
corresponding diminution in the teeth. In the catalogue of the College
of Surgeons Museum, there is appended to the entry which identifies a
Blenheim Spaniel's skull, the words—“the teeth are closely crowded
together,” and to the entry concerning the skull of a King Charles's
Spaniel the words—“the teeth are closely packed, p. 3, is placed quite
transversely to the axis of the skull.” It is further noteworthy that in
a case where there is no diminished use of the jaws, but where they have
been shortened by selection, a like want of concomitant variation is
manifested: the case being that of the bull-dog, in the upper jaw of
which also, “the premolars ... are excessively crowded, and placed
obliquely or even transversely to the long axis of the skull.”[1]
If, then, in cases where we can test it, we find no concomitant
variation in co-operative parts that are near together—if we do not find
it in parts which, though belonging to different tissues, are so closely
united as teeth and jaws—if we do not find it even when the co-operative
parts are not only closely united, but are formed out of the same
tissue, like the crab's eye and its peduncle; what shall we say of
co-operative parts which, besides being composed of different tissues,
are remote from one another? Not only are we forbidden to assume that
they vary together, but we are warranted in asserting that they can have
no tendency to vary together. And what are the implications in cases
where increase of a structure can be of no service unless there is
concomitant increase in many distant structures, which have to join it
in performing the action for which it is useful?
As far back as 1864 (_Principles of Biology_, § 166) I named in
illustration an animal carrying heavy horns—the extinct Irish elk; and
indicated the many changes in bones, muscles, blood-vessels, nerves,
composing the fore-part of the body, which would be required to make an
increment of size in such horns advantageous. Here let me take another
instance—that of the giraffe: an instance which I take partly because,
in the sixth edition of the _Origin of Species_, issued in 1872, Mr.
Darwin has referred to this animal when effectually disposing of certain
arguments urged against his hypothesis. He there says:—
“In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable that several other
parts should be modified and co-adapted. Although every part of the
body varies slightly, it does not follow that the necessary parts
should always vary in the right direction and to the right degree”
(p. 179).
And in the summary of the chapter, he remarks concerning the adjustments
in the same quadruped, that “the prolonged use of all the parts together
with inheritance will have aided in an important manner in their
co-ordination” (p. 199): a remark probably having reference chiefly to
the increased massiveness of the lower part of the neck; the increased
size and strength of the thorax required to bear the additional burden;
and the increased strength of the fore-legs required to carry the
greater weight of both. But now I think that further consideration
suggests the belief that the entailed modifications are much more
numerous and remote than at first appears; and that the greater part of
these are such as cannot be ascribed in any degree to the selection of
favourable variations, but must be ascribed exclusively to the inherited
effects of changed functions. Whoever has seen a giraffe gallop will
long remember the sight as a ludicrous one. The reason for the
strangeness of the motions is obvious. Though the fore limbs and the
hind limbs differ so much in length, yet in galloping they have to keep
pace—must take equal strides. The result is that at each stride, the
angle which the hind limbs describe round their centre of motion is much
larger than the angle described by the fore limbs. And beyond this, as
an aid in equalizing the strides, the hind part of the back is at each
stride bent very much downwards and forwards. Hence the hind-quarters
appear to be doing nearly all the work. Now a moment's observation shows
that the bones and muscles composing the hind-quarters of the giraffe,
perform actions differing in one or other way and degree, from the
actions performed by the homologous bones and muscles in a mammal of
ordinary proportions, and from those in the ancestral mammal which gave
origin to the giraffe. Each further stage of that growth which produced
the large fore-quarters and neck, entailed some adapted change in sundry
of the numerous parts composing the hind-quarters; since any failure in
the adjustment of their respective strengths would entail some defect in
speed and consequent loss of life when chased. It needs but to remember
how, when continuing to walk with a blistered foot, the taking of steps
in such a modified way as to diminish pressure on the sore point, soon
produces aching of muscles which are called into unusual action, to see
that over-straining of any one of the muscles of the giraffe's
hind-quarters might quickly incapacitate the animal when putting out all
its powers to escape; and to be a few yards behind others would cause
death. Hence if we are debarred from assuming that co-operative parts
vary together even when adjacent and closely united—if we are still more
debarred from assuming that with increased length of fore-legs or of
neck, there will go an appropriate change in any one muscle or bone in
the hind-quarters; how entirely out of the question it is to assume that
there will simultaneously take place the appropriate changes in _all_
those many components of the hind-quarters which severally require
re-adjustment. It is useless to reply that an increment of length in the
fore-legs or neck might be retained and transmitted to posterity,
waiting an appropriate variation in a particular bone or muscle in the
hind-quarters, which, being made, would allow of a further increment.
For besides the fact that until this secondary variation occurred the
primary variation would be a disadvantage often fatal; and besides the
fact that before such an appropriate secondary variation might be
expected in the course of generations to occur, the primary variation
would have died out; there is the fact that the appropriate variation of
one bone or muscle in the hind-quarters would be useless without
appropriate variations of all the rest—some in this way and some in
that—a number of appropriate variations which it is impossible to
suppose.
Nor is this all. Far more numerous appropriate variations would be
indirectly necessitated. The immense change in the ratio of
fore-quarters to hind-quarters would make requisite a corresponding
change of ratio in the appliances carrying on the nutrition of the two.
The entire vascular system, arterial and veinous, would have to undergo
successive unbuildings and rebuildings to make its channels everywhere
adequate to the local requirements; since any want of adjustment in the
blood-supply in this or that set of muscles, would entail incapacity,
failure of speed, and loss of life. Moreover the nerves supplying the
various sets of muscles would have to be proportionately changed; as
well as the central nervous tracts from which they issued. Can we
suppose that all these appropriate changes, too, would be step by step
simultaneously made by fortunate spontaneous variations, occurring along
with all the other fortunate spontaneous variations? Considering how
immense must be the number of these required changes, added to the
changes above enumerated, the chances against any adequate
re-adjustments fortuitously arising must be infinity to one.
If the effects of use and disuse of parts are inheritable, then any
change in the fore parts of the giraffe which affects the action of the
hind limbs and back, will simultaneously cause, by the greater or less
exercise of it, a re-moulding of each component in the hind limbs and
back in a way adapted to the new demands; and generation after
generation the entire structure of the hind-quarters will be
progressively fitted to the changed structure of the fore-quarters: all
the appliances for nutrition and innervation being at the same time
progressively fitted to both. But in the absence of this inheritance of
functionally-produced modifications, there is no seeing how the required
re-adjustments can be made.
* * * * *
Yet a third class of difficulties stands in the way of the belief that
the natural selection of useful variations is the sole factor of organic
evolution. This class of difficulties, already pointed out in § 166 of
the _Principles of Biology_, I cannot more clearly set forth than in the
words there used. Hence I may perhaps be excused for here quoting them.
“Where the life is comparatively simple, or where surrounding
circumstances render some one function supremely important, the
survival of the fittest may readily bring about the appropriate
structural change, without any aid from the transmission of
functionally-acquired modifications. But in proportion as the life
grows complex—in proportion as a healthy existence cannot be secured
by a large endowment of some one power, but demands many powers; in
the same proportion do there arise obstacles to the increase of any
particular power, by “the preservation of favoured races in the
struggle for life.” As fast as the faculties are multiplied, so fast
does it become possible for the several members of a species to have
various kinds of superiorities over one another. While one saves its
life by higher speed, another does the like by clearer vision,
another by keener scent, another by quicker hearing, another by
greater strength, another by unusual power of enduring cold or
hunger, another by special sagacity, another by special timidity,
another by special courage; and others by other bodily and mental
attributes. Now it is unquestionably true that, other things equal,
each of these attributes, giving its possessor an extra chance of
life, is likely to be transmitted to posterity. But there seems no
reason to suppose that it will be increased in subsequent
generations by natural selection. That it may be thus increased, the
individuals not possessing more than average endowments of it, must
be more frequently killed off than individuals highly endowed with
it; and this can happen only when the attribute is one of greater
importance, for the time being, than most of the other attributes.
If those members of the species which have but ordinary shares of
it, nevertheless survive by virtue of other superiorities which they
severally possess; then it is not easy to see how this particular
attribute can be developed by natural selection in subsequent
generations. The probability seems rather to be, that by
gamogenesis, this extra endowment will, on the average, be
diminished in posterity—just serving in the long run to compensate
the deficient endowments of other individuals, whose special powers
lie in other directions; and so to keep up the normal structure of
the species. The working out of the process is here somewhat
difficult to follow; but it appears to me that as fast as the number
of bodily and mental faculties increases, and as fast as the
maintenance of life comes to depend less on the amount of any one,
and more on the combined action of all; so fast does the production
of specialities of character by natural selection alone, become
difficult. Particularly does this seem to be so with a species so
multitudinous in its powers as mankind; and above all does it seem
to be so with such of the human powers as have but minor shares in
aiding the struggle for life—the æsthetic faculties, for example.”
Dwelling for a moment on this last illustration of the class of
difficulties described, let us ask how we are to interpret the
development of the musical faculty. I will not enlarge on the family
antecedents of the great composers. I will merely suggest the inquiry
whether the greater powers possessed by Beethoven and Mozart, by Weber
and Rossini, than by their fathers, were not due in larger measure to
the inherited effects of daily exercise of the musical faculty by their
fathers, than to inheritance, with increase, of spontaneous variations;
and whether the diffused musical powers of the Bach clan, culminating in
those of Johann Sebastian, did not result in part from constant
practice; but I will raise the more general question—How came there that
endowment of musical faculty which characterizes modern Europeans at
large, as compared with their remote ancestors. The monotonous chants of
low savages cannot be said to show any melodic inspiration; and it is
not evident that an individual savage who had a little more musical
perception than the rest, would derive any such advantage in the
maintenance of life as would secure the spread of his superiority by
inheritance of the variation. And then what are we to say of harmony? We
cannot suppose that the appreciation of this, which is relatively
modern, can have arisen by descent from the men in whom successive
variations increased the appreciation of it—the composers and musical
performers; for on the whole, these have been men whose worldly
prosperity was not such as enabled them to rear many children inheriting
their special traits. Even if we count the illegitimate ones, the
survivors of these added to the survivors of the legitimate ones, can
hardly be held to have yielded more than average numbers of descendants;
and those who inherited their special traits have not often been thereby
so aided in the struggle for existence as to further the spread of such
traits. Rather the tendency seems to have been the reverse.
Since the above passage was written, I have found in the second volume
of _Animals and Plants under Domestication_, a remark made by Mr.
Darwin, practically implying that among creatures which depend for their
lives on the efficiency of numerous powers, the increase of any one by
the natural selection of a variation is necessarily difficult. Here it
is.
“Finally, as indefinite and almost illimitable variability is the
usual result of domestication and cultivation, with the same part or
organ varying in different individuals in different or even in
directly opposite ways; and as the same variation, if strongly
pronounced, usually recurs only after long intervals of time, any
particular variation would generally be lost by crossing, reversion,
and the accidental destruction of the varying individuals, unless
carefully preserved by man.”—Vol. ii, 292.
Remembering that mankind, subject as they are to this domestication and
cultivation, are not, like domesticated animals, under an agency which
picks out and preserves particular variations; it results that there
must usually be among them, under the influence of natural selection
alone, a continual disappearance of any useful variations of particular
faculties which may arise. Only in cases of variations which are
specially preservative, as for example, great cunning during a
relatively barbarous state, can we expect increase from natural
selection alone. We cannot suppose that minor traits, exemplified among
others by the æsthetic perceptions, can have been evolved by natural
selection. But if there is inheritance of functionally-produced
modifications of structure, evolution of such minor traits is no longer
inexplicable.
* * * * *
Two remarks made by Mr. Darwin have implications from which the same
general conclusion must, I think, be drawn. Speaking of the variability
of animals and plants under domestication, he says:—
“Changes of any kind in the conditions of life, even extremely
slight changes, often suffice to cause variability.... Animals and
plants continue to be variable for an immense period after their
first domestication; ... In the course of time they can be
habituated to certain changes, so as to become less variable; ...
There is good evidence that the power of changed conditions
accumulates; so that two, three, or more generations must be exposed
to new conditions before any effect is visible.... Some variations
are induced by the direct action of the surrounding conditions on
the whole organization, or on certain parts alone, and other
variations are induced indirectly through the reproductive system
being affected in the same manner as is so common with organic
beings when removed from their natural conditions.”—(_Animals and
Plants under Domestication_, vol. ii, 270.)
There are to be recognized two modes of this effect produced by changed
conditions on the reproductive system, and consequently on offspring.
Simple arrest of development is one. But beyond the variations of
offspring arising from imperfectly developed reproductive systems in
parents—variations which must be ordinarily in the nature of
imperfections—there are others due to a changed balance of functions
caused by changed conditions. The fact noted by Mr. Darwin in the above
passage, “that the power of changed conditions accumulates; so that two,
three, or more generations must be exposed to new conditions before any
effect is visible,” implies that during these generations there is going
on some change of constitution consequent on the changed proportions and
relations of the functions. I will not dwell on the implication, which
seems tolerably clear, that this change must consist of such
modifications of organs as adapt them to their changed functions; and
that if the influence of changed conditions “accumulates,” it must be
through the inheritance of such modifications. Nor will I press the
question—What is the nature of the effect registered in the reproductive
elements, and which is subsequently manifested by variations?—Is it an
effect entirely irrelevant to the new requirements of the variety?—Or is
it an effect which makes the variety less fit for the new
requirements?—Or is it an effect which makes it more fit for the new
requirements? But not pressing these questions, it suffices to point out
the necessary implication that changed functions of organs _do_, in some
way or other, register themselves in changed proclivities of the
reproductive elements. In face of these facts it cannot be denied that
the modified action of a part produces an inheritable effect—be the
nature of that effect what it may.
The second of the remarks above adverted to as made by Mr. Darwin, is
contained in his sections dealing with correlated variations. In the
_Origin of Species_, p. 114, he says—
“The whole organization is so tied together during its growth and
development, that when slight variations in any one part occur, and
are accumulated through natural selection, other parts become
modified.”
And a parallel statement contained in _Animals and Plants under
Domestication_, vol. ii, p. 320, runs thus—
“Correlated variation is an important subject for us; for when one
part is modified through continued selection, either by man or under
nature, other parts of the organization will be unavoidably
modified. From this correlation it apparently follows that, with our
domesticated animals and plants, varieties rarely or never differ
from each other by some single character alone.”