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 fundament 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 vertebrae; 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 vertebrae, 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 connexions
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 connexion 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 favourable ways. We are enabled to understand such marvellous
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 colours and markings by a species which is
structurally quite different--so simulated that even a practised
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_, Sec. 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 fossae 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."[41]
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_, Sec. 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 Sec. 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 aesthetic 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 aesthetic 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."
By what process does a changed part modify other parts? By modifying
their functions in some way or degree, seems the necessary answer. It is
indeed, imaginable, that where the part changed is some dermal appendage
which, becoming larger, has abstracted more of the needful material from
the general stock, the effect may consist simply in diminishing the
amount of this material available for other dermal appendages, leading
to diminution of some or all of them, and may fail to affect in
appreciable ways the rest of the organism: save perhaps the
blood-vessels near the enlarged appendage. But where the part is an
active one--a limb, or viscus, or any organ which constantly demands
blood, produces waste matter, secretes, or absorbs--then all the other
active organs become implicated in the change. The functions performed
by them have to constitute a moving equilibrium; and the function of one
cannot, by alteration of the structure performing it, be modified in
degree or kind, without modifying the functions of the rest--some
appreciably and others inappreciably, according to the directness or
indirectness of their relations. Of such inter-dependent changes, the
normal ones are naturally inconspicuous; but those which are partially
or completely abnormal, sufficiently carry home the general truth. Thus,
unusual cerebral excitement affects the excretion through the kidneys in
quantity or quality or both. Strong emotions of disagreeable kinds check
or arrest the flow of bile. A considerable obstacle to the circulation
offered by some important structure in a diseased or disordered state,
throwing more strain upon the heart, causes hypertrophy of its muscular
walls; and this change which is, so far as concerns the primary evil, a
remedial one, often entails mischiefs in other organs. "Apoplexy and
palsy, in a scarcely credible number of cases, are directly dependent on
hypertrophic enlargement of the heart." And in other cases, asthma,
dropsy, and epilepsy are caused. Now if a result of this
inter-dependence as seen in the individual organism, is that a local
modification of one part produces, by changing their functions,
correlative modifications of other parts, then the question here to be
put is--Are these correlative modifications, when of a kind falling
within normal limits, inheritable or not. If they are inheritable, then
the fact stated by Mr. Darwin that "when one part is modified through
continued selection," "other parts of the organization will be
unavoidably modified" is perfectly intelligible: these entailed
secondary modifications are transmitted _pari passu_ with the successive
modifications produced by selection. But what if they are not
inheritable? Then these secondary modifications caused in the
individual, not being transmitted to descendants, the descendants must
commence life with organizations out of balance, and with each increment
of change in the part affected by selection, their organizations must
get more out of balance--must have a larger and larger amounts of
re-organization to be made during their lives. Hence the constitution of
the variety must become more and more unworkable.
The only imaginable alternative is that the re-adjustments are effected
in course of time by natural selection. But, in the first place, as we
find no proof of concomitant variation among directly co-operative parts
which are closely united, there cannot be assumed any concomitant
variation among parts which are both indirectly co-operative and far
from one another. And, in the second place, before all the many
required re-adjustments could be made, the variety would die out from
defective constitution. Even were there no such difficulty, we should
still have to entertain a strange group of propositions, which would
stand as follows:--1. Change in one part entails, by reaction on the
organism, changes, in other parts, the functions of which are
necessarily changed. 2. Such changes worked in the individual, affect,
in some way, the reproductive elements: these being found to evolve
unusual structures when the constitutional balance has been continuously
disturbed. 3. But the changes in the reproductive elements thus caused,
are not such as represent these functionally-produced changes: the
modifications conveyed to offspring are irrelevant to these various
modifications functionally produced in the organs of the parents. 4.
Nevertheless, while the balance of functions cannot be re-established
through inheritance of the effects of disturbed functions on structures,
wrought throughout the individual organism; it can be re-established by
the inheritance of fortuitous variations which occur in all the affected
organs without reference to these changes of function.
Now without saying that acceptance of this group of propositions is
impossible, we may certainly say that it is not easy.
* * * * *
"But where are the direct proofs that inheritance of
functionally-produced modifications takes place?" is a question which
will be put by those who have committed themselves to the current
exclusive interpretation. "Grant that there are difficulties; still,
before the transmitted effects of use and disuse can be legitimately
assigned in explanation of them, we must have good evidence that the
effects of use and disuse _are_ transmitted."
Before dealing directly with this demurrer, let me deal with it
indirectly, by pointing out that the lack of recognized evidence may be
accounted for without assuming that there is not plenty of it.
Inattention and reluctant attention lead to the ignoring of facts which
really exist in abundance; as is well illustrated in the case of
pre-historic implements. Biassed by the current belief that no traces of
man were to be found on the Earth's surface, save in certain superficial
formations of very recent date, geologists and anthropologists not only
neglected to seek such traces, but for a long time continued to
pooh-pooh those who said they had found them. When M. Boucher de Perthes
at length succeeded in drawing the eyes of scientific men to the flint
implements discovered by him in the quarternary deposits of the Somme
valley; and when geologists and anthropologists had thus been convinced
that evidences of human existence were to be found in formations of
considerable age, and thereafter began to search for them; they found
plenty of them all over the world. Or again, to take an instance closely
germane to the matter, we may recall the fact that the contemptuous
attitude towards the hypothesis of organic evolution which naturalists
in general maintained before the publication of Mr. Darwin's work,
prevented them from seeing the multitudinous facts by which it is
supported. Similarly, it is very possible that their alienation from the
belief that there is a transmission of those changes of structure which
are produced by changes of action, makes naturalists slight the evidence
which supports that belief and refuse to occupy themselves in seeking
further evidence.
If it be asked how it happens that there have been recorded
multitudinous instances of variations fortuitously arising and
re-appearing in offspring, while there have not been recorded instances
of the transmission of changes functionally produced, there are three
replies. The first is that changes of the one class are many of them
conspicuous, while those of the other class are nearly all
inconspicuous. If a child is born with six fingers, the anomaly is not
simply obvious but so startling as to attract much notice; and if this
child, growing up, has six-fingered descendents, everybody in the
locality hears of it. A pigeon with specially-coloured feathers, or one
distinguished by a broadened and upraised tail, or by a protuberance of
the neck, draws attention by its oddness; and if in its young the trait
is repeated, occasionally with increase, the fact is remarked, and there
follows the thought of establishing the peculiarity by selection. A lamb
disabled from leaping by the shortness of its legs, could not fail to be
observed; and the fact that its offspring were similarly short-legged,
and had a consequent inability to get over fences, would inevitably
become widely known. Similarly with plants. That this flower had an
extra number of petals, that that was unusually symmetrical, and that
another differed considerably in colour from the average of its kind,
would be easily seen by an observant gardener; and the suspicion that
such anomalies are inheritable having arisen, experiments leading to
further proofs that they are so, would frequently be made. But it is not
thus with functionally-produced modifications. The seats of these are in
nearly all cases the muscular, osseous, and nervous systems, and the
viscera--parts which are either entirely hidden or greatly obscured.
Modification in a nervous centre is inaccessible to vision; bones may be
considerably altered in size or shape without attention being drawn to
them; and, covered with thick coats as are most of the animals open to
continuous observation, the increases or decreases in muscles must be
great before they become externally perceptible.
A further important difference between the two inquiries is that to
ascertain whether a fortuitous variation is inheritable, needs merely a
little attention to the selection of individuals and the observation of
offspring; while to ascertain whether there is inheritance of a
functionally-produced modification, it is requisite to make arrangements
which demand the greater or smaller exercise of some part or parts;
and it is difficult in many cases to find such arrangements, troublesome
to maintain them even for one generation, and still more through
successive generations.
Nor is this all. There exist stimuli to inquiry in the one case which do
not exist in the other. The money-interest and the interest of the
fancier, acting now separately and now together, have prompted
multitudinous individuals to make experiments which have brought out
clear evidence that fortuitous variations are inherited. The
cattle-breeders who profit by producing certain shapes and qualities;
the keepers of pet animals who take pride in the perfections of those
they have bred; the florists, professional and amateur, who obtain new
varieties and take prizes; form a body of men who furnish naturalists
with countless of the required proofs. But there is no such body of men,
led either by pecuniary interest or the interest of a hobby, to
ascertain by experiments whether the effects of use and disuse are
inheritable.
Thus, then, there are amply sufficient reasons why there is a great deal
of direct evidence in the one case and but little in the other: such
little being that which comes out incidentally. Let us look at what
there is of it.
* * * * *
Considerable weight attaches to a fact which Brown-Sequard discovered,
quite by accident, in the course of his researches. He found that
certain artificially-produced lesions of the nervous system, so small
even as a section of the sciatic nerve, left, after healing, an
increasing excitability which ended in liability to epilepsy; and there
afterwards came out the unlooked-for result that the offspring of
guinea-pigs which had thus acquired an epileptic habit such that a pinch
on the neck would produce a fit, inherited an epileptic habit of like
kind. It has, indeed, been since alleged that guinea pigs tend to
epilepsy, and that phenomena of the kind described, occur where there
have been no antecedents like those in Brown-Sequard's case. But
considering the improbability that the phenomena observed by him
happened to be nothing more than phenomena which occasionally arise
naturally, we may, until there is good proof to the contrary, assign
some value to his results.
Evidence not of this directly experimental kind, but nevertheless of
considerable weight, is furnished by other nervous disorders. There is
proof enough that insanity admits of being induced by circumstances
which, in one or other way, derange the nervous functions--excesses of
this or that kind; and no one questions the accepted belief that
insanity is inheritable. Is it alleged that the insanity which is
inheritable is that which spontaneously arises, and that the insanity
which follows some chronic perversion of functions is not inheritable?
This does not seem a very reasonable allegation; and until some warrant
for it is forthcoming, we may fairly assume that there is here a further
support for belief in the transmission of functionally-produced changes.
Moreover, I find among physicians the belief that nervous disorders of a
less severe kind are inheritable. Men who have prostrated their nervous
systems by prolonged overwork or in some other way, have children more
or less prone to nervousness. It matters not what may be the form of
inheritance--whether it be of a brain in some way imperfect, or of a
deficient blood-supply; it is in any case the inheritance of
functionally-modified structures.
Verification of the reasons above given for the paucity of this direct
evidence, is yielded by contemplation of it; for it is observable that
the cases named are cases which, from one or other cause, have thrust
themselves on observation. They justify the suspicion that it is not
because such cases are rare that many of them cannot be cited; but
simply because they are mostly unobtrusive, and to be found only by that
deliberate search which nobody makes. I say nobody, but I am wrong.
Successful search has been made by one whose competence as an observer
is beyond question, and whose testimony is less liable than that of all
others to any bias towards the conclusion that such inheritance takes
place. I refer to the author of the _Origin of Species_.
* * * * *
Now-a-days most naturalists are more Darwinian than Mr. Darwin himself.
I do not mean that their beliefs in organic evolution are more decided;
though I shall be supposed to mean this by the mass of readers, who
identify Mr. Darwin's great contribution to the theory of organic
evolution, with the theory of organic evolution itself, and even with
the theory of evolution at large. But I mean that the particular factor
which he first recognized as having played so immense a part in organic
evolution, has come to be regarded by his followers as the sole factor,
though it was not so regarded by him. It is true that he apparently
rejected altogether the causal agencies alleged by earlier inquirers. In
the Historical Sketch prefixed to the later editions of his _Origin of
Species_ (p. xiv, note), he writes:--"It is curious how largely my
grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous
grounds of opinion of Lamarck in his 'Zoonomia' (vol. i, pp. 500-510),
published in 1794." And since, among the views thus referred to, was the
view that changes of structure in organisms arise by the inheritance of
functionally-produced changes, Mr. Darwin seems, by the above sentence,
to have implied his disbelief in such inheritance. But he did not mean
to imply this; for his belief in it as a cause of evolution, if not an
important cause, is proved by many passages in his works. In the first
chapter of the _Origin of Species_ (p. 8 of the sixth edition), he says
respecting the inherited effects of habit, that "with animals the
increased use or disuse of parts has had a more marked influence;" and
he gives as instances the changed relative weights of the wing bones and
leg bones of the wild duck and the domestic duck, "the great and
inherited development of the udders in cows and goats," and the drooping
ears of various domestic animals. Here are other passages taken from the
latest edition of the work.
"I think there can be no doubt that use in our domestic animals has
strengthened and enlarged certain parts, and disuse diminished
them; and that such modifications are inherited" (p. 108). [And on
the following pages he gives five further examples of such
effects.] "Habit in producing constitutional peculiarities and use
in strengthening and disuse in weakening and diminishing organs,
appear in many cases to have been potent in their effects" (p.
131). "When discussing special cases, Mr. Mivart passes over the
effects of the increased use and disuse of parts, which I have
always maintained to be highly important, and have treated in my
'Variation under Domestication' at greater length than, as I
believe, any other writer" (p. 176). "Disuse, on the other hand,
will account for the less developed condition of the whole inferior
half of the body, including the lateral fins" (p. 188). "I may give
another instance of a structure which apparently owes its origin
exclusively to use or habit" (p. 188). "It appears probable that
disuse has been the main agent in rendering organs rudimentary"
(pp. 400-401). "On the whole, we may conclude that habit, or use
and disuse, have, in some cases, played a considerable part in the
modification of the constitution and structure; but that the
effects have often been largely combined with, and sometimes
overmastered by, the natural selection of innate variations" (p.
114).
In his subsequent work, _The Variation of Animals and Plants under
Domestication_, where he goes into full detail, Mr. Darwin gives more
numerous illustrations of the inherited effects of use and disuse. The
following are some of the cases, quoted from volume i of the first
edition.
Treating of domesticated rabbits, he says:--"the want of exercise
has apparently modified the proportional length of the limbs in
comparison with the body" (p. 116). "We thus see that the most
important and complicated organ [the brain] in the whole
organization is subject to the law of decrease in size from disuse"
(p. 129). He remarks that in birds of the oceanic islands "not
persecuted by any enemies, the reduction of their wings has
probably been caused by gradual disuse." After comparing one of
these, the water-hen of Tristan d'Acunha, with the European
water-hen, and showing that all the bones concerned in flight are
smaller, he adds--"Hence in the skeleton of this natural species
nearly the same changes have occurred, only carried a little
further, as with our domestic ducks, and in this latter case I
presume no one will dispute that they have resulted from the
lessened use of the wings and the increased use of the legs" (pp.
286-7). "As with other long-domesticated animals, the instincts of
the silk-moth have suffered. The caterpillars, when placed on a
mulberry-tree, often commit the strange mistake of devouring the
base of the leaf on which they are feeding, and consequently fall
down; but they are capable, according to M. Robinet, of again
crawling up the trunk. Even this capacity sometimes fails, for M.
Martins placed some caterpillars on a tree, and those which fell
were not able to remount and perished of hunger; they were even
incapable of passing from leaf to leaf" (p. 304).
Here are some instances of like meaning from volume ii.
"In many cases there is reason to believe that the lessened use of
various organs has affected the corresponding parts in the
offspring. But there is no good evidence that this ever follows in
the course of a single generation.... Our domestic fowls, ducks,
and geese have almost lost, not only in the individual but in the
race, their power of flight; for we do not see a chicken, when
frightened, take flight like a young pheasant.... With domestic
pigeons, the length of the sternum, the prominence of its crest,
the length of the scapulae and furcula, the length of the wings as
measured from tip to tip of the radius, are all reduced relatively
to the same parts in the wild pigeon." [After detailing kindred
diminutions in fowls and ducks, Mr. Darwin adds] "The decreased
weight and size of the bones, in the foregoing cases, is probably
the indirect result of the reaction of the weakened muscles on the
bones" (pp. 297-8). "Nathusius has shown that, with the improved
races of the pig, the shortened legs and snout, the form of the
articular condyles of the occiput, and the position of the jaws
with the upper canine teeth projecting in a most anomalous manner
in front of the lower canines, may be attributed to these parts not
having been fully exercised.... These modifications of structure,
which are all strictly inherited, characterise several improved
breeds, so that they cannot have been derived from any single
domestic or wild stock. With respect to cattle, Professor Tanner
has remarked that the lungs and liver in the improved breeds 'are
found to be considerably reduced in size when compared with those
possessed by animals having perfect liberty;' ... The cause of the
reduced lungs in highly-bred animals which take little exercise is
obvious" (pp. 299-300). [And on pp. 301, 302 and 303, he gives
facts showing the effects of use and disuse in changing, among
domestic animals, the characters of the ears, the lengths of the
intestines, and, in various ways, the natures of the instincts.]
But Mr. Darwin's admission, or rather his assertion, that the
inheritance of functionally-produced modifications has been a factor in
organic evolution, is made clear not by these passages alone and by
kindred ones. It is made clearer still by a passage in the preface to
the second edition of his _Descent of Man_. He there protests against
that current version of his views in which this factor makes no
appearance. The passage is as follows.
"I may take this opportunity of remarking that my critics
frequently assume that I attribute all changes of corporeal
structure and mental power exclusively to the natural selection of
such variations as are often called spontaneous; whereas, even in
the first edition of the 'Origin of Species,' I distinctly stated
that great weight must be attributed to the inherited effects of
use and disuse, with respect both to the body and mind."
Nor is this all. There is evidence that Mr. Darwin's belief in the
efficiency of this factor, became stronger as he grew older and
accumulated more evidence. The first of the extracts above given, taken
from the sixth edition of the _Origin of Species_, runs thus:--
"I think there can be no doubt that use in our domestic animals has
strengthened and enlarged certain parts, and disuse diminished
them; and that such modifications are inherited."
Now on turning to the first edition, p. 134, it will be found that
instead of the words--"I think there can be no doubt," the words
originally used were--"I think there can be _little_ doubt." That this
deliberate erasure of a qualifying word and substitution of a word
implying unqualified belief, was due to a more decided recognition of a
factor originally under-estimated, is clearly implied by the wording of
the above-quoted passage from the preface to the _Descent of Man_; where
he says that "_even_ in the first edition of the 'Origin of Species,'"
&c.: the implication being that much more in subsequent editions, and
subsequent works, had he insisted on this factor. The change thus
indicated is especially significant as having occurred at a time of life
when the natural tendency is towards fixity of opinion.
During that earlier period when he was discovering the multitudinous
cases in which his own hypothesis afforded solutions, and simultaneously
observing how utterly futile in these multitudinous cases was the
hypothesis propounded by his grandfather and Lamarck, Mr. Darwin was,
not unnaturally, almost betrayed into the belief that the one is
all-sufficient and the other inoperative. But in the mind of one so
candid and ever open to more evidence, there naturally came a reaction.
The inheritance of functionally-produced modifications, which, judging
by the passage quoted above concerning the views of these earlier
enquirers, would seem to have been at one time denied, but which as we
have seen was always to some extent recognized, came to be recognized
more and more, and deliberately included as a factor of importance.
* * * * *
Of this reaction displayed in the later writings of Mr. Darwin, let us
now ask--Has it not to be carried further? Was the share in organic
evolution which Mr. Darwin latterly assigned to the transmission of
modifications caused by use and disuse, its due share? Consideration of
the groups of evidences given above, will, I think, lead us to believe
that its share has been much larger than he supposed even in his later
days.
There is first the implication yielded by extensive classes of phenomena
which remain inexplicable in the absence of this factor. If, as we see,
co-operative parts do not vary together, even when few and close
together, and may not therefore be assumed to do so when many and
remote, we cannot account for those innumerable changes in organization
which are implied when, for advantageous use of some modified part, many
other parts which join it in action have to be modified.
Further, as increasing complexity of structure, accompanying increasing
complexity of life, implies increasing number of faculties, of which
each one conduces to preservation of self or descendants; and as the
various individuals of a species, severally requiring something like the
normal amounts of all these, may individually profit, here by an unusual
amount of one, and there by an unusual amount of another; it follows
that as the number of faculties becomes greater, it becomes more
difficult for any one to be further developed by natural selection. Only
where increase of some one is _predominantly_ advantageous does the
means seem adequate to the end. Especially in the case of powers which
do not subserve self-preservation in appreciable degrees, does
development by natural selection appear impracticable.
It is a fact recognized by Mr. Darwin, that where, by selection through
successive generations, a part has been increased or decreased, its
reaction upon other parts entails changes in them. This reaction is
effected through the changes of function involved. If the changes of
structure produced by such changes of function, are inheritable, then
the re-adjustment of parts throughout the organism, taking place
generation after generation, maintains an approximate balance; but if
not, then generation after generation the organism must get more and
more out of gear, and tend to become unworkable.
Further, as it is proved that change in the balance of functions
registers its effects on the reproductive elements, we have to choose
between the alternatives that the registered effects are irrelevant to
the particular modifications which the organism has undergone, or that
they are such as tend to produce repetitions of these modifications. The
last of these alternatives makes the facts comprehensible; but the first
of them not only leaves us with several unsolved problems, but is
incongruous with the general truth that by reproduction, ancestral
traits, down to minute details, are transmitted.
Though, in the absence of pecuniary interests and the interests in
hobbies, no such special experiments as those which have established the
inheritance of fortuitous variations have been made to ascertain whether
functionally-produced modifications are inherited; yet certain apparent
instances of such inheritance have forced themselves on observation
without being sought for. In addition to other indications of a less
conspicuous kind, is the one I have given above--the fact that the
apparatus for tearing and mastication has decreased with decrease of its
function, alike in civilized man and in some varieties of dogs which
lead protected and pampered lives. Of the numerous cases named by Mr.
Darwin, it is observable that they are yielded not by one class of parts
only, but by most if not all classes--by the dermal system, the muscular
system, the osseous system, the nervous system, the viscera; and that
among parts liable to be functionally modified, the most numerous
observed cases of inheritance are furnished by those which admit of
preservation and easy comparison--the bones: these cases, moreover,
being specially significant as showing how, in sundry unallied species,
parallel changes of structure have occurred along with parallel changes
of habit.
What, then, shall we say of the general implication? Are we to stop
short with the admission that inheritance of functionally-produced
modifications takes place only in cases in which there is evidence of
it? May we properly assume that these many instances of changes of
structure caused by changes of function, occurring in various tissues
and various organs, are merely special and exceptional instances having
no general significance? Shall we suppose that though the evidence which
already exists has come to light without aid from a body of inquirers,
there would be no great increase were due attention devoted to the
collection of evidence? This is, I think, not a reasonable supposition.
To me the _ensemble_ of the facts suggests the belief, scarcely to be
resisted, that the inheritance of functionally-produced modifications
takes place universally. Looking at physiological phenomena as
conforming to physical principles, it is difficult to conceive that a
changed play of organic forces which in many cases of different kinds
produces an inherited change of structure, does not do this in all
cases. The implication, very strong I think, is that the action of every
organ produces on it a reaction which, usually not altering its rate of
nutrition, sometimes leaves it with diminished nutrition consequent on
diminished action, and at other times increases its nutrition in
proportion to its increased action; that while generating a modified
_consensus_ of functions and of structures, the activities are at the
same time impressing this modified _consensus_ on the sperm-cells and
germ-cells whence future individuals are to be produced; and that in
ways mostly too small to be identified, but occasionally in more
conspicuous ways and in the course of generations, the resulting
modifications of one or other kind show themselves. Further, it seems to
me that as there are certain extensive classes of phenomena which are
inexplicable if we assume the inheritance of fortuitous variations to be
the sole factor, but which become at once explicable if we admit the
inheritance of functionally-produced changes, we are justified in
concluding that this inheritance of functionally-produced changes has
been not simply a co-operating factor in organic evolution, but has been
a co-operating factor without which organic evolution, in its higher
forms at any rate, could never have taken place.
Be this or be it not a warrantable conclusion, there is, I think, good
reason for a provisional acceptance of the hypothesis that the effects
of use and disuse are inheritable; and for a methodic pursuit of
inquiries with the view of either establishing it or disproving it. It
seems scarcely reasonable to accept without clear demonstration, the
belief that while a trivial difference of structure arising
spontaneously is transmissible, a massive difference of structure,
maintained generation after generation by change of function, leaves no
trace in posterity. Considering that unquestionably the modification of
structure by function is a _vera causa_, in so far as concerns the
individual; and considering the number of facts which so competent an
observer as Mr. Darwin regarded as evidence that transmission of such
modifications takes place in particular cases; the hypothesis that such
transmission takes place in conformity with a general law, holding of
all active structures, should, I think, be regarded as at least a good
working hypothesis.
* * * * *
But now supposing the broad conclusion above drawn to be
granted--supposing all to agree that from the beginning, along with
inheritance of useful variations fortuitously arising, there has been
inheritance of effects produced by use and disuse; do there remain no
classes of organic phenomena unaccounted for? To this question I think
it must be replied that there do remain classes of organic phenomena
unaccounted for. It may, I believe, be shown that certain cardinal
traits of animals and plants at large are still unexplained; and that a
further factor must be recognized. To show this, however, will require
another paper.