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 descendants, 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-Séquard 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-Séquard'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. 11 of the first edition), he says
respecting the inherited effects of habit, that “with animals the
increased use or disuse of parts has had a 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 scapulæ 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.