THE INHERITANCE OF ACQUIRED CHARACTERS AS A FACTOR IN EVOLUTION
Lamarck’s Theory
One of the most striking and peculiar characteristics of living things
is that through use a part is able to carry out a particular function
better than before, and in some cases the use of the part leads to its
increase in size. Conversely, disuse leads to the decrease of a part in
size. We are perfectly familiar with this process in ourselves as
applied to our nervous system and muscles.
It is not surprising that the idea should have arisen that, if the
results of the use of a part are inherited by the next generation, the
adaptation of organisms might be explained in this way. The presence of
the organs of touch, in those parts of the body that are more likely to
come into contact with foreign bodies, offers a striking parallel to the
perfecting of the sensation of touch that can be brought about through
the use of any part. The development of eyes only on the exposed parts
of the body, as on the tentacles of the sedentary annelids, or along the
margin of the mantle of a bivalve mollusk, suggests that there may be
some direct connection between their presence in these regions and the
effect of light on the parts. In fact, ever since the time of Lamarck,
there have been many zoologists who have claimed that many of the
adaptations of organisms have arisen in this way, that is, through the
inheritance of the characters acquired through use. In general this
theory is summed up in the phrase, “the inheritance of acquired
characters.”
This view is prominently associated with the name of Lamarck, who held,
however, a different view in regard to the origin of some of the other
structures of the organism. Moreover, Erasmus Darwin, even before
Lamarck, had suggested the principle of the inheritance of acquired
characters.
As has just been said, Lamarck held that the inheritance of acquired
characters was only one of the ways in which animals have become
changed, and he clearly stated that in the case of all plants and of
some of the lower animals the change (evolution) which he supposed them
to undergo was due to the general influence of the environment. Since
plants and the lower animals (as he supposed) have no central nervous
system, or at least no such well-defined nervous system as have the
higher animals, Lamarck thought that they could not have evolved in the
same way as have the higher animals. We now know that, so far as the
lower animals, at least, are concerned, there was no need for such a
distinction, since many of their responses are like those of the higher
animals. This distinction that Lamarck made is responsible, no doubt,
for a misconception that was long held in regard to a part of his views.
It is often stated that he supposed the desire of the animal for a
particular part has led to the development of that part; while in
reality he only maintained the desire to use a particular organ to
fulfil some want led to its better development through exercise, and the
result was inherited. Lamarck also supposed that the _decrease_ in use
of a part which leads to its decrease in size accounts for the
degeneration of organs.
Lamarck first advanced his theory in 1801, when he cited the following
examples in its favor. A bird, driven through want to the water to find
its food, will separate its toes when they strike the water. The skin
uniting the bases of the toes will be stretched in consequence, and in
this way the broad membrane between the toes of ducks and geese has been
acquired. The toes of a bird that is in the habit of perching on a tree
become elongated in consequence of becoming stretched, hence has arisen
the foot with the long toes characteristic of arboreal birds.
Shore-birds, “which do not care to swim,” but must approach the water in
order to obtain food, will be in danger of sinking into the mud, “but,
wishing to act so that their body shall not fall into the liquid, they
will contract the habit of extending and lengthening their legs.” Hence
have arisen the stiltlike legs of shore-birds.
These ideas were more fully elaborated in the following year. He added
the further examples: Our dray-horses have arisen through the use to
which they have been put, and the race-horse also, which has been used
in a different way. Cultivated plants, on the contrary, are the result
of the new environment to which they have been subjected.
In the “Philosophic Zoologique,” published in 1809, Lamarck has much
more fully developed his theory. Here he combats strenuously the idea
that species are fixed. His point of view may be judged by the following
propositions, which he believes can be established:—
1. That all organized bodies of our globe are veritable productions of
nature, which she has successively produced in the course of a long
time.
2. That in her progress nature began, and begins still every day, to
produce the simplest organisms, and that she still produces directly the
same primitive kinds of organizations. This process has been called
spontaneous generation.
3. That the first beginning of animals and of plants takes place in
favorable localities and under favorable circumstances. An organic
movement having once established their production, they have of
necessity gradually developed their organs, and have become diversified
in the course of time.
4. That the power of growth of each part of the body being inherited as
a consequence of the first effect of life, different modes of
multiplication and of regeneration have arisen, and these have been
conserved.
5. That with the aid of sufficient time and of favorable circumstances
the changes that have taken place on the surface of the globe have
called forth new structures and new habits, and in consequence have
modified the organs of the body, and made animals and plants such as we
see them at the present day.
6. Finally, as a result of these changes that living bodies have been
forced to undergo, species have been formed, but these species have only
a relative constancy, and are not as ancient as is nature herself. If
the environment remains the same, species also remain the same, as is
exemplified by the animals living at present in Egypt, which are exactly
like those living there in ancient times.
Lamarck concludes that the appearance of stability is always mistaken by
the layman for the reality, because, in general, every one judges things
relatively to himself. In fact, species are not absolutely constant, but
are so only temporarily. “The influence of the environment is continuous
and always active, but its effects may only be recognized after a long
time.” The irregularity and the complexity of the organization of
animals is the outcome of the infinitely diversified circumstances to
which they have been subjected. These changes, Lamarck claims, do not
directly cause modifications in the form of animals,[17] but bring about
changes in their needs, and changes in their needs bring about changes
in their actions. If the needs remain the same, the acquired actions
become habits. These habitual actions lead to the use of certain parts
in preference to others, and this in turn to an alteration in form and
structure. The individuals so changed breed together and leave
descendants that inherit the acquired modification.
Footnote 17:
This is clearly meant to be applied only in the case of higher
animals.
Curiously enough, Lamarck follows up this argument by citing some cases
amongst plants that have been changed directly by the action of the
environment. He says that since plants have no motions they have
consequently no habits, but they are developed by changes in their
nutrition, etc., and this brings about the superiority of some of the
vital movements over others.
Amongst domestic animals Lamarck cites the case of the dog, that has
come from a wild form like the wolf, but having been carried into
different countries has acquired different and new habits, and this has
led to the formation of new races, such as the bulldog, greyhound,
pug-dog, spaniel, etc.
Lamarck’s argument shifts so often back and forth from animals to
plants, that it is clear that in his own mind he did not see any
important difference between the action of the environment on plants,
and the use of the organs of the animal. He gives in this same
connection his oft-quoted summary of what he calls the two laws of
nature “which observation always establishes.”
First Law. In every animal, that has not passed beyond the term of its
development, the frequent and sustained use of any organ strengthens it,
develops it, increases its size, and gives it strength proportionate to
the length of time of its employment. On the other hand, the continued
lack of use of the same organ sensibly weakens it; it deteriorates, and
its faculties diminish progressively until at last it disappears.
Second Law. Nature preserves everything that she has caused the
individual to acquire or to lose by the influence of the circumstances
to which the race has been for a long time exposed, and consequently by
the influence of the predominant use of certain organs (or in
consequence of its continued disuse). She does this by the generation of
new individuals which are produced with the newly acquired organs. This
occurs, provided that the acquired changes were common to the two sexes,
or to the individuals that produced the new forms.
These laws are, Lamarck says, fundamental truths which cannot be
misunderstood except by those who have never observed or followed nature
in her operations. He insists that it is a mistake to suppose that the
parts are responsible for the functions, for it is easy to demonstrate
that it is the needs and uses of the organs that have caused the parts
to develop.
If it is supposed, he continues, that these laws are hypothetical, they
may be demonstrated by the following facts: The adult baleen whale is
without teeth, although in the fœtus teeth are present, concealed in the
jaws. The loss of the teeth is the result of the whale swallowing its
food without first masticating it. The ant-eater is also without teeth,
and has also the habit of swallowing its food without chewing it. The
mole has very small eyes, and this is the result of its having made very
little use of them, since its habits are subterranean. Another animal,
the aspalax, has only the rudiments of eyes, and has almost completely
lost the power of sight. This animal also lives underground like the
mole.
Proteus, an aquatic salamander living in deep caves, has only
rudimentary eyes. In these latter cases it is the disuse of the eye that
has led to its degeneration. This is proven, Lamarck adds, by the fact
that the organs of hearing are never in this condition, because sound
vibrations penetrate everywhere, even into the densest bodies.
It is a part of the plan of organization of the reptiles that they have
four legs; but the snakes, although belonging to this group, have no
legs. This absence of legs is explained by their having acquired the
habit of gliding over the ground, and of concealing themselves in the
grass. Owing to their repeated effort to elongate themselves, in order
to pass through narrow spaces, their bodies have become drawn out. Under
these circumstances legs would be useless, since long ones would
interfere with their motion, and short ones could not move their long
bodies. Since the plan of organization limits the snakes to only four
legs, and since this number would be useless, they have disappeared.
Many insects are destitute of wings, although wings are a part of the
plan of organization of this group. They are absent only in those forms
whose habits render wings useless, consequently they have disappeared
through disuse.
The preceding cases are those in which the disuse of an organ has led to
its degeneration. The following cases are cited to show that by use an
organ increases in size. The formation of the web in the feet of
water-birds has already been given as a character which Lamarck supposes
to have been acquired through use; also the case of shore-birds, which,
by an effort to elongate their legs, have actually made them so in the
course of time. The necks of water-birds are also long on account of
their having been stretched in the efforts to catch fish. The long
tongues of the ant-eater, of the woodpecker, and of humming-birds are
the result of use, and the long, forked tongue of serpents has come from
their using their tongue to feel objects in front of them.
Fishes that have acquired the habit of living in shallow water,
flounders, soles, etc., have been forced to swim on their sides in order
to approach nearer to the shore. Since more light comes from above than
from below, the eye on the under side, straining to turn to the light,
has finally migrated to the upper side.
The habit of eating great quantities of food, which distends the
digestive organs, has caused the bodies of herbivorous quadrupeds to
become large, as seen in the elephant, the rhinoceros, oxen, horses, and
buffaloes. The habit of standing for a long time on their feet has
caused some animals to develop hard, thick hoofs. Herbivorous animals,
that inhabit countries where they are constantly subjected to attack, as
deer and antelopes for example, are forced to escape by rapid flight,
and in consequence their bodies have become slenderer and their legs
thinner. The horns, antlers, and protuberances that many of these
animals possess are the results of their butting each other when
angered.
“The long neck and the form of the giraffe offer a curious case. We know
that the giraffe is the tallest of all animals. It inhabits the centre
of Africa, living in those localities where the earth is nearly always
dry and without herbage. It is obliged to browse on the foliage of
trees, and this leads to its stretching continually upwards. As a result
of this habit, carried on for a long time, in all the individuals of the
race, the anterior limbs have become longer than the posterior, and its
neck has also lengthened, so that the giraffe without rising on its
hind-legs stretches up its neck and can reach to the height of six
metres.”
The curved claws of the carnivora have arisen from the necessity of
grasping their prey. The power of retracting the claws has also been
acquired by the effort to draw them in when running over hard ground.
The abdominal pouch of the kangaroo, in which the young are carried,
opens anteriorly, and this has led to the animal standing erect so that
its young are not injured. In consequence, the fore-legs have become
shorter through disuse, and the hind-legs have become stronger through
use. The tail, which is also used as a support, has become enormously
thick at its base.
The sloth has been compelled to seek refuge in the trees, and has taken
up its abode permanently there, feeding on leaves. Its movements are
limited to those involved in crawling along the limbs in order to reach
the leaves. After feeding it remains inactive and sluggish, these habits
being provoked by the heat of the climate. The results of its mode of
life have been to cause the arms to become elongated due to the habit of
the sloth of grasping the limbs of the tree; the claws of the fingers
and toes have also become long and hooked in order to retain their hold.
The digits that do not make any individual movements have lost the power
to do so, and have become fused, and can only be bent in and
straightened out. The thighs, being bent out to clasp the larger
branches, have caused the pelvis to widen, and, in consequence, the
cotyloid cavities have become directed backward. Many of the bones of
the skeleton have become fused, as a result of the immobility of the
animal.
Lamarck says, that “Nature, in producing, successively, all the species
of animals, beginning with the most imperfect, or the most simple, and
terminating with the most perfect, has gradually complicated their
organization. These animals becoming scattered throughout the habitable
regions of the globe each species has received from the influences of
its surroundings its present habits, and the modifications of the parts
the use of which we recognize.”
Such are Lamarck’s views and a fairly complete statement of the facts
from which he draws his conclusions. His illustrations appear naïve, and
often not a little ludicrous, but it must be admitted that, despite
their absurdities, his theory appears in some cases to account
wonderfully well for the facts. The long legs of wading birds, the long
neck and disproportionately long fore-legs of the giraffe, the structure
of the sloth, and particularly the degeneration of the eyes of animals
living in the dark, seem to find a simple explanation in the principle
of the inheritance of acquired characters. But the crucial point of the
entire theory is passed over in silence, or rather is taken for granted
by Lamarck, namely, the inheritance in the offspring of the characters
acquired through use or disuse in the parent. He does not even discuss
this topic, but in several places states unreservedly that the increase
or decrease of a part reappears in the next generation. It is here that
Lamarck’s theory has been attacked in more modern times, for as soon as
experimental proof was demanded to show that the results of use or of
disuse of an organ is inherited, no such proof was forthcoming. Yet the
theory is one that has the great merit of being capable of experimental
test, and it is astonishing to find that, with the immense amount that
has been written by his followers, so few attempts have been made to
give the theory a thorough test. The few results that have been obtained
are not, however, favorable to the theory, but almost the only attempts
at experiment that have been made in this direction have been those of
mutilating certain parts; and were it not for popular belief to the
effect that such mutilations are inherited, one would least expect to
get evidence for or against the theory in this direction. Lamarck
himself believed that the changes were slowly acquired, and I think
modern Lamarckians are justified in claiming that the validity of the
theory can only be tested by experiments in which the organism is
subjected to influences extending over a considerable period, although
Lamarck appears to have believed that the first results may appear quite
soon. Before expressing any opinion in regard to the probability of the
theory, let us examine what the followers of Lamarck have contributed in
the way of evidence to the theory, rather than the applications that
they have made of the theory. We shall also find it profitable to
consider some of the modern criticism, to which the theory has been
subjected.
Despite the contempt with which Darwin referred to Lamarck’s theory, he
himself, as we have seen, often made use of the principle of the
inheritance of acquired characters, and even employed the same
illustrations cited by Lamarck. Darwin seems to have misunderstood
Lamarck’s view, and to have accepted the current opinion that Lamarck
supposed an animal acquired a new organ by desiring or needing it.
Darwin says, “Heaven forefend me from Lamarck’s nonsense of a tendency
to progressive adaptation from the slow willing of the animals.” Darwin
speaks of Lamarck as stating that animals will that the egg shall be a
particular form so as to become attached to particular objects.
Lamarck’s latest biographer, Packard, says he is unable to find any
statements of this sort in Lamarck’s writings.
The following cases that Darwin tried to explain through the inheritance
of acquired characters are exactly like those to which Lamarck applied
his theory. The bones of the wing of the domestic duck weigh less than
those of the wild duck, and the bones of the leg more. Darwin believes
this is due to the effects of the inheritance of acquired characters.
The drooping ears of many domestic mammals are also explained by him as
a result of disuse—“the animals being seldom much alarmed.” In speaking
of the male of the beetle, _Onites apelles_, Darwin quotes Kirby to the
effect that the tarsi are so habitually lost that the species has been
described without this part of the foot. In the sacred beetle of Egypt
the tarsus is totally absent. Hence he concludes that the absence of
tarsi in the sacred beetle, and the rudimentary condition of the tarsus
in others, is probably the result of disuse, rather than a case of
inheritance of a mutilation. Darwin grants that “the evidence that
accidental mutilations can be inherited is at present not decisive, but
the remarkable case observed by Brown-Séquard in guinea-pigs of the
inherited effects of operations should make us cautious in denying this
tendency.”
The wingless condition of several insects inhabiting oceanic islands has
come about, Darwin thinks, through disuse. The ostrich also, owing to
its increase in size, made less use of its wings and more use of its
legs, with the result that its wings degenerated and its legs got
stronger. The rudimentary condition of the eyes of the mole is the
result of disuse, “aided perhaps by natural selection.” Many of the
animals inhabiting the caves of Kentucky and of Carniola are blind, and
this is ascribed to disuse. “As it is difficult to imagine that the
eyes, though useless, could be in any way injurious to animals living in
darkness, their loss may be attributed to disuse.” The long neck of the
giraffe Darwin attributes partly to natural selection and partly to use.
These references will suffice to show that Darwin is in full accord with
the main argument of Lamarck. In fact, the curious hypothesis of
pangenesis that Darwin advanced was invented partly to account for the
inheritance of acquired characters. Despite the hesitancy that Darwin
himself felt in advancing this view, and contrary to Huxley’s advice, he
at last published his provisional hypothesis of pangenesis in the
twenty-seventh chapter of his “Animals and Plants under Domestication.”
Darwin’s Hypothesis of Pangenesis
The study of bud variation, of the various forms of inheritance, and of
reproduction and of the causes of variation, led him, Darwin says, to
the belief that these subjects stand in some sort of relation to each
other. He says: “I have been led, or rather forced, to form a view which
to a certain extent connects these facts by a tangible method. Every one
would wish to explain to himself, even in an imperfect manner, how it is
possible for a character possessed by some remote ancestor suddenly to
reappear in the offspring; how the effects of increased or decreased use
of a limb can be transmitted to the child; how the male sexual element
can act not solely on the ovules, but occasionally on the mother form;
how a hybrid can be produced by the union of the cellular tissue of two
plants independently of the organs of generation; how a limb can be
reproduced on the exact line of amputation, with neither too much nor
too little added; how the same organism may be produced by such widely
different processes, as budding and true seminal generation; and,
lastly, how of two allied forms, one passes in the course of its
development through the most complex metamorphoses, and the other does
not do so, though when mature both are alike in every detail of
structure. I am aware that my view is merely a provisional hypothesis or
speculation; but, until a better one be advanced, it will serve to bring
together a multitude of facts which are at present left disconnected by
any efficient cause.”
In presenting the hypothesis of pangenesis Darwin begins by enumerating
the different kinds of sexual and asexual processes of reproduction, for
which he hopes to offer a provisional explanation. Here we find
mentioned various methods of budding and self-division, regeneration,
parthenogenesis, sexual reproduction, and the inheritance of acquired
characters. It is with the last only that we are here chiefly concerned;
in fact, the need of an hypothesis _of this sort_ to explain the other
kinds of inheritance is by no means evident. There are, however, two
other phenomena, besides that of the supposed inheritance of acquired
characters, to which the hypothesis of pangenesis might appear to apply
specially, namely, the effect of foreign pollen on the tissues of the
mother plant, and the supposed influence of the union with the first
male on the subsequent young (telegony). It is, however, far from being
shown that any influence of this latter kind really occurs, despite the
fact that it is generally believed in by breeders.
It is important to observe that Darwin proposes to explain on the
hypothesis of pangenesis, not only the inheritance of characters
acquired through use, but also the decrease of structures through
disuse; and this applies, not only to the structure, but to function as
well, as when the intelligence of the dog is explained through his
association with man, and the tameness of the domestic rabbits through
their long confinement. In the following quotation these points are
referred to: “How can the use or disuse of a particular limb or of the
brain affect a small aggregate of reproductive cells, seated in a
distant part of the body, in such a manner that the being developed from
these cells inherits the characters of either one or both parents? Even
an imperfect answer to this question would be satisfactory.”
Coming now to the theory, we find that it consists of one chief
assumption and several minor ones. “It is universally admitted that the
cells or units of the body increase by self-division or proliferation,
retaining the same nature, and that they ultimately become converted
into the various tissues and substances of the body. But besides this
means of increase I assume that the units throw off minute granules
which are dispersed throughout the whole system; that these, when
supplied with proper nutriment, multiply by self-division, and are
ultimately developed into units like those from which they were
originally derived. These granules may be called gemmules. They are
collected from all parts of the system to constitute the sexual
elements, and their development in the next generation forms a new
being; but they are likewise capable of transmission in a dormant state
to future generations, and may then be developed.... Gemmules are
supposed to be thrown off by every unit, not only during the adult
state, but during each stage of development of every organism; but not
necessarily during the continued existence of the same unit. Lastly, I
assume that the gemmules in their dormant state have a mutual affinity
for each other, leading to their aggregation into buds, or into the
sexual elements. Hence, it is not the reproductive organs, or buds,
which generate new organisms, but the units of which each individual is
composed. These assumptions constitute the provisional hypothesis which
I have called Pangenesis.”
It will be noticed that the first assumption is that the cells throw off
minute gemmules or granules. The second assumption is that these are
collected in the reproductive organs, or in buds, or in regenerating
parts; the third assumption is that the gemmules may lie dormant through
several generations; the fourth, that the development of the
reproductive cells is not so much the development of the cell itself,
but of the gemmules that have collected in it. The fifth assumption is
that the gemmules are thrown off at all stages of development; the
sixth, that in their dormant state they have a mutual affinity for each
other; the seventh, that there may be a sort of continual competition in
the germ-cells between the original gemmules and the new ones, and,
according to which win, the old or the new form develops. Thus we see on
closer analysis that the pangenesis hypothesis is made up of a goodly
number of different assumptions. At least half a dozen imaginary
properties are ascribed to the imaginary gemmules, and these attributes
are all essential to the working of the hypothesis.
Some of the more obvious objections to the hypothesis have been stated
by Darwin himself. Such, for instance, as our ignorance at what stage in
their history the body-cells are capable of throwing off gemmules, and
whether they collect only at certain times in the reproductive organs,
as the increased flow of blood to these organs at certain seasons might
seem to indicate. Nor have we any evidence that they are carried by the
blood at all. The experiment of Galton, of transfusing the blood of one
animal into another, and finding that this produced no effect on the
young that were born later, might be interpreted to mean that gemmules
are not transported by the blood; but this kind of experiment is
inconclusive, especially in the light of recent results on the effect of
the blood of one animal on that of another.
A part of the evidence on which Darwin relied to support his theory has
been shown to be incorrect by later work. Thus the assumption that more
than a single pollen grain, or more than one spermatozoon, is necessary
in some cases for fertilization, is certainly wrong. In most cases, in
fact, the entrance of more than one spermatozoon into the egg is
disastrous to the development. The cases referred to by Darwin can
probably be explained by the difficulty that some of the pollen grains,
or spermatozoa, may have in penetrating the egg, or to the immaturity or
impotence of some of the male germ-cells, and not to the need of more
than one to accomplish the true fertilization.
Darwin’s idea that the small number of gemmules in the unfertilized egg
may account for the lack of power of such eggs to develop until they are
fertilized, has been shown to be incorrect by recent results in
experimental embryology. We now know that many different kinds of
stimuli have the power to start the development of the egg. Moreover, we
also know that if a single spermatozoon is supplied with a piece of
egg-protoplasm without a nucleus, it suffices to cause this piece of
protoplasm to develop.
In the case of regeneration, which Darwin also tries to explain on the
pangenesis hypothesis, we find that there is no need at all for an
hypothesis of this sort; and there are a number of facts in connection
with regeneration that are not in harmony with the hypothesis. For
instance, when a part is cut off, the same part is regenerated; but
under these circumstances it cannot be imagined that the part removed
supplies the gemmules for the new part. Darwin tries to meet this
objection by the assumption that every part of the body contains
gemmules from every other part. But it has been shown that if a limb of
the newt is completely extirpated, a new limb does not regenerate; and
there is no reason why it should not do so on Darwin’s assumption that
germs of the limb exist throughout the body.
The best-authenticated cases of the influence of the male on the tissues
of the female are those in plants, where one species, or variety, is
fertilized by another. Thus, if the orange is fertilized by the pollen
of the lemon, the fruit may have the color and flavor of the lemon. Now
the fruit is a product of the tissues of the ovary of the female, and
not a part of the seedling that develops in the fruit from the
cross-fertilized egg-cell. Analogous cases are recorded for the bean,
whose pods may have their color influenced by fertilizing the flower
with pollen of another variety having pods of a different color. In
these cases we do not know whether the color of the fruit is influenced
directly by the foreign pollen, or whether the influence is through the
embryo that develops from the egg-cell. The action may appear to be the
same, however, in either case; but because it seems probable here that
there is some sort of influence of one tissue on another, let us not too
readily conclude that this is brought about through any such imaginary
bodies as gemmules. It may be directly caused, for instance, by some
chemical substance produced in the young hybrid plant. If this is the
case, the result would not be different in kind from that of certain
flowers whose color may be influenced by certain chemical substances in
the soil.
In the cases amongst animals, where the maternal tissues are believed to
be influenced by a previous union with the male, as in the oft-cited
case of Lord Morton’s mare, a reëxamination of the evidence by Ewart has
shown that the case is not demonstrated, and not even probable. Several
years ago I tried to test this view in the case of mice. A white mouse
was first bred to a dark male house-mouse, and the next time to a white
mouse, but none of the offspring from the second union showed any trace
of black. If the spermatozoa of the dark mouse are hypodermically
injected into the body-cavity of the female, the subsequent young from a
white male show no evidence that the male cells have had any influence
on the ovary.
The following facts, spoken of by Darwin himself, are not in favor of
his hypothesis of pangenesis: “But it appears at first sight a fatal
objection to our hypothesis that a part of an organ may be removed
during several successive generations, and if the operation be not
followed by disease, the lost part reappears in the offspring. Dogs and
horses formerly had their tails docked during many generations without
any inherited effect; although, as we have seen, there is some reason to
believe that the tailless conditions of certain sheep-dogs is due to
such inheritance.” The answer that Darwin gives is that the gemmules
themselves, that were once derived from the part, are still present in
other parts of the body, and it is from these that the organs in the
next generation may be derived. But Darwin fails to point out that, if
this were the case, it must also be true for those cases in which an
organ is no longer used. Its decrease in size in successive generations
cannot be due to its disuse, for the rest of the body would supply the
necessary gemmules to keep it at its full state of development. Thus, in
trying to meet an obvious objection to his hypothesis, Darwin brings
forward a new view that is fatal to another part of his hypothesis.
The following cases, also given by Darwin, are admitted by him to be
inexplicable on his hypothesis: “With respect to variations due to
reversion, there is a similar difference between plants propagated from
buds and seeds. Many varieties can be propagated securely by buds, but
generally or invariably revert to their parent forms by seed. So, also,
hybridized plants can be multiplied to any extent by buds, but are
continually liable to reversion by seed,—that is, to the loss of their
hybrid or intermediate character. I can offer no satisfactory
explanation of these facts. Plants with variegated leaves, phloxes with
striped flowers, barberries with seedless fruit, can all be securely
propagated by buds taken from the stem or branches; but buds from the
roots of these plants almost invariably lose their character and revert
to their former condition. This latter fact is also inexplicable, unless
buds developed from the roots are as distinct from those on the stem, as
is one bud on the stem from another, and we know that these latter
behave like independent organisms.” As Darwin here states, these facts
appear to be directly contradictory to his hypothesis, and he makes no
effort to account for them.
The entire question of the possibility of the inheritance of acquired
characters is itself at present far from being on a satisfactory basis,
as we shall try to show; and Darwin’s attempt at an explanation, in his
chapter on pangenesis, does not put the matter in a much more
satisfactory condition.
The Neo-Lamarckian School
Let us now turn our attention to a school that has grown up in modern
times, the members of which call themselves Neo-Lamarckians. Let us see
if they have supplied the essential evidence that is required to
establish the Lamarckian view, namely, that characters acquired by the
individual are transmitted to the offspring.
Lamarck’s views were adopted by Herbert Spencer, and play an important
rôle in his “Principles of Biology” (1866-1871), and even a more
conspicuous part in his later writings. In the former he cites, amongst
other cases, that of “a puppy taken from its mother at six weeks old
who, although never taught ‘to beg’ (an accomplishment his mother had
been taught), spontaneously took to begging for everything he wanted
when about seven or eight months old.” If tricks like this are
inheritable is it not surprising that more puppies do not stand on their
hind-legs?
The larger hands of the laboring classes in England are supposed to be
inherited by their children, and the smaller hands of the leisure
classes are supposed to be the result of the disuse of the hands by
their ancestors; but even if these statements in regard to size are
true, there are many other conceivable causes that may have led to this
result.
Short-sightedness appears more often, it is said, in those classes of
society that make most use of their eyes in reading and in writing; but
if we ask for experimental evidence to show that this is due to
inheritance, and not due to the children spoiling their eyes at school,
there is none forthcoming. The problem is by no means so simple as the
uninitiated may be led to believe.
Spencer thinks that “some of the best illustrations of functional
heredity are furnished by mental characteristics.” He cites the musical
faculty as one that could not have been acquired by natural selection,
and must have arisen through the inheritance of acquired modifications.
The explanation offered is “that the habitual association of certain
cadences of speech with certain emotions has clearly established in the
race an organized and inherited connection between such cadences and
such emotions, ... and that by the continued hearing and practice of
melody there has been gained and transmitted an increasing musical
sensibility.” But a statement that the results have been acquired in
this way does not supply the proof which the theory is in need of;
neither does it follow that, because the results cannot be explained by
the theory of natural selection, therefore, they must be explained by
the Lamarckian theory.
The clearest proofs that Spencer finds of the inheritance of acquired
characters are in the well-known experiments of Brown-Séquard. These
experiments will be more fully discussed below. Amongst the other morbid
processes that Spencer thinks furnish evidence in favor of this view,
are cases of a tendency to gout, the occurrence of mental tricks,
musical prodigies, liability to consumption, in all of which cases the
fundamental distinction between the inheritance of an acquired character
and the inherited tendency toward a particular malady is totally
ignored.
Twenty-seven years later (in 1893) Spencer took up the open challenge of
the anti-Lamarckian writers, and by bringing forward a number of new
_arguments_ attempted to reinstate the principle of the inheritance of
acquired characters. His first illustration is drawn from the
distribution of the sense of touch in different parts of our bodies.
Weber’s experiments have shown that if the sharp points of a pair of
compasses are applied to the tips of the forefingers, the sensation of
two separate points is given when the points are only one-twelfth of an
inch apart, and if the points are moved nearer together, they give the
sensation of only one point. The inner surfaces of the second joints of
the fingers can only distinguish two points when they are one-sixth of
an inch apart. The innermost joints are less discriminating, and are
about equal in the power of discrimination to the tip of the nose. The
end of the big toe, the palm of the hand, and the cheek discriminate
only about one-fifth as well as do the tips of the fingers. The back of
the hand and the top of the head distinguish only about one-fifteenth as
well as the finger-tips. The front of the thigh, near the knee, is
somewhat less sensitive than the back of the hand. On the breast the
points of the compasses must be separated by more than an inch and a
half in order to give two sensations. In the middle of the back the
points must be separated by two and a half inches, or more, in order to
give two separate impressions.
What is the meaning of these differences, Spencer asks. If natural
selection has brought about the result, then it must be shown that
“these degrees of endowment have advantaged the possessor to such an
extent that not infrequently life has been directly or indirectly
preserved by it.” He asks if this, or anything approaching this, result
could have occurred.
That the superior perceptiveness of the forefinger-tip might have arisen
through selection is admitted by Spencer, but how could this have been
the case, he asks, for the middle of the back, and for the face? The tip
of the nose has three times more power of discrimination than the lower
part of the forehead. Why should the front of the thigh near the knee be
twice as perceptive as in the middle of the thigh; and why should the
middle of the back and of the neck and the middle of the forearm and of
the thigh stand at such low levels? Is it possible, Spencer asks again,
that natural selection has determined these relations, and if not, how
can they be explained? His reply is that the differences can all be
accounted for on the theory of the inheritance of use, for it is evident
that “these gradations in tactile perceptiveness correspond with the
gradations in the tactual exercise of the parts.” Except from contact
with the clothing the body receives hardly any touch sensations from
outside, and this accounts for its small power of discrimination. The
greater sensitiveness of the chest and abdomen, as compared with the
back, is due to these regions being more frequently touched by the
hands, and is also owing to inheritance from more remote ancestors, in
which the lower surface of the body was more likely to have come in
contact with foreign objects than was the back. The middle of the
forearm and of the thigh are also less exposed than the knee and the
hand, and have correspondingly the power of tactile discrimination less
well developed.
Weber showed that the tip of the tongue is more sensitive than any other
part of the body, for it can distinguish between two points only one
twenty-fourth of an inch apart. Obviously, Spencer says, natural
selection cannot account for such extreme delicacy of touch, because,
even if it were useful for the tongue to distinguish objects by touch,
this power could never be of vital importance to the animal. It cannot
even be supposed that such delicacy is necessary for the power of
speech.
The sensitiveness of the tongue can be accounted for, however, Spencer
claims, as the result of the constant use of the tongue in exploring the
cavity of the mouth. It is continually moving about, and touching now
one part, and now another, of the mouth cavity. “No advantage is gained.
It is simply that the tongue’s position renders perpetual exploration
almost inevitable.” No other explanation of the facts seemed possible to
Spencer.
Two questions will at once suggest themselves. First, can it be shown
that the sensitiveness to touch in various parts of the body is the
result of individual experience? Have we learned to discriminate in
those parts of the body that are most often brought into contact with
surrounding objects? Even the power of discrimination in the tips of the
fingers can be improved, as Spencer himself has shown, in the case of
the blind, and of skilled compositors. Can we account in this way for
the power of discrimination in various parts of the body? In other
words, if, beginning in infancy, the middle of the back constantly came
into contact with surrounding objects, would this region become as
sensitive as the tips of the fingers? The experiment has not, of course,
been carried out, but it is not probable that it would succeed. I
venture this opinion on the ground of the relative number of the nerves
and of the organs of touch on the back, as compared with those of the
finger-tips. But, it will be asked, will not the number of the
sense-organs become greater if a part is continually used by the
individual? It is improbable that much improvement could be brought
about in this way. The improvement that takes place through experience
is probably not so much the result of the development of more
sense-organs, as of better discrimination in the sensation, because the
increased power can be very quickly acquired.
An examination of the relative abundance of touch-spots in the skin
shows that they are much more numerous in regions of greater
sensitiveness. The following table, taken from Sherrington’s account of
sense-organs in Schaefer’s “Textbook of Physiology,” gives the smallest
distance that two points, simultaneously applied, can be recognized as
such (and not simply as one impression) in different regions.
Mm.
Tip of tongue 1.1
Volar surface of 2.3
ungual phalanx of
finger
Red surface of lip 4.5
Volar face of second 4.5
phalanx
Dorsal face of third 6.8
phalanx
Side of tongue 9.0
Third line of tongue, 9.0
27 mm. from tip
Plantar face of ungual 11.3
phalanx of first toe
Palm 11.3
Back of second phalanx 11.3
of finger
Forehead 22.6
Back of ankle 22.6
Back of hand 31.6
Forearm, leg 40.6
Dorsum of foot 40.6
Outer sternum 45.1
Back of neck 54.1
Middle of back 67.1
Upper arm, thigh 67.1
The great difference in the sensitiveness of the skin in the different
regions is very striking, and if, as seems probable, about the same
proportionate difference is found at birth, then the degree of
sensibility of the different regions is inborn, and is not the result of
each individual experience. Until it can be shown that more of the
sense-organs develop in any special part, as the result of the increased
use of the part, we have no real basis on which to establish, even as
probable, the Lamarckian view.
But, after all, is the distribution of the sense-organs exactly that
which we should expect on the Lamarckian theory? Has not Spencer taken
too much for granted in this direction? The lower part of the forearm
(represented by 15) we should expect to be more sensitive than the
protected surface of the eyelid (11.3), but this is not the case. The
forehead (22.6) is much less sensitive than the forearm, and only half
as sensitive as the eyelid. The knee (36.1) is still less sensitive than
any of these other parts, and this does not in the least accord with the
theory, since in its constant moving forward it must be continually
coming into contact with foreign bodies. The fact that the back is as
insensitive as the upper arm (67.7) can hardly be accredited in favor of
the theory. The great difference between the lower third of the forearm
on the ulnar surface (15) and the upper arm (67.7) seems out of all
proportion to what we should expect on the theory. And is it not a
little odd that the end of the nose should be so highly sensitive?
There is another point that we cannot afford to neglect in this
connection. It is known that in addition to touch-spots there are warm
and cold spots in the skin, which produce, when touched, the sensation
of warmth, or of cold, respectively, and not the sensation of touch. The
degree of sensitiveness of different regions of the body throws an
interesting side-light on Spencer’s argument.
The warm spots are much fewer than the cold spots. The spots are
arranged in short lines radiating from centres which coincide with
hairs. The number of these spots varies a good deal, even in the same
region of the skin. If the sensitiveness of the skin is tested, the
following results will be obtained. The list includes twelve grades of
sensitiveness, beginning with the places giving the lowest maximum of
intensity. About one hundred square areas were tested in each region.
COLD SENSATIONS
1. Tips of fingers and toes, malleoli, ankle.
2. Other parts of digits, tip of nose, olecranon.
3. Glabella, chin, palm, gums.
4. Occiput, patella, wrist.
5. Clavicle, neck, forehead, tongue.
6. Buttocks, upper eyelid.
7. Lower eyelid, popliteal space, sole, cheek.
8. Inner aspect of thigh, arm above elbow.
9. The intercostal spaces along axillary line.
10. Mammary areola.
11. Nipple, flank.
12. Certain areas of the loins and abdomen.
WARMTH SENSATIONS
0. Lower gum, mucosa of cheek, cornea.
1. Tips of fingers and toes, cavity of mouth, conjunctiva, and
patella.
2. Remaining surface of digits, middle of forehead, olecranon.
3. Glabella, chin, clavicle.
4. Palm, buttock, popliteal space.
5. Neck.
6. Back.
7. Lower eyelid, cheek.
8. Nipple, loin.
These two tables show the great differences in the range of
sensitiveness to cold and to warmth in different parts of the body. I
doubt if any one will attempt to show that these differences of range of
sensation can be accounted for either by natural selection or by the
Lamarckian hypothesis.
Of course, it does not necessarily follow that, because this is true for
the warm and cold spots, that it must also be true for the tactile
organs; but I think that the fact of such a great difference in the
responsiveness to cold and to warmth in different parts of the body
should put us on our guard against a too ready acceptation of Spencer’s
argument. More especially is this seen to be necessary, when, as has
been shown above, the distribution of the touch-organs themselves by no
means closely corresponds to what we should expect, if they have
developed in response to contact, as Spencer maintains.
The other main argument advanced by Spencer to fortify the theory of the
inheritance of acquired characters, and at the same time to show the
inadequacy of the theory of natural selection, is based on the idea of
what he calls the “coöperation of the parts” that is required in order
to carry out any special act. Spencer contends that “the relative powers
of coöperative parts cannot be adjusted solely by the survival of the
fittest, and especially where the parts are numerous and the coöperation
complex.”
Spencer illustrates his point by the case of the extinct Irish elk,
whose immensely developed horns weighed over a hundredweight. The horns,
together with the massive skull, could not have been supported by the
outstretched neck without many and great changes of the muscles and
bones of the neck and of the fore-part of the body. Unless, for
instance, the fore-legs had been also strengthened, there would be
failure in fighting and in locomotion. Since “we cannot assume
spontaneous increase of all these parts proportionate to the additional
strains, we cannot suppose them to increase by variations one at once,
without supposing the creature to be disadvantaged by the weight and
nutrition of the parts that were for a time useless,—parts, moreover,
which would revert to their original sizes before the other needful
variations occurred.”
The answer made to this argument was that coördinating parts vary
together. In reply to which Spencer points to the following cases, which
show that this is not so: The blind crayfish in the Kentucky caves have
lost their eyes, but not the stalks that carry them. Again, the normal
relation between the length of tongue and of beak in some varieties of
pigeons is lost. The greater decrease in the jaws in some species of pet
dogs than of the number of their teeth has caused the teeth to become
crowded.[18] “I then argued that if coöperative parts, small in number,
and so closely associated as these are, do not vary together, it is
unwarrantable to allege that coöperative parts, which are very numerous
and remote from one another, vary together.” Spencer puts himself here
into the position of seriously maintaining that, because some
coöperative parts do not vary together, therefore no coöperative parts
have ever done so, and he has taken this position in the face of some
well-known cases in which certain parts have been found to vary
together.
Footnote 18:
It is curious that Spencer does not see that this case is as much
against his point as in favor of it, since the _unused_ teeth did not
also degenerate.
In this same connection Spencer brings up the familiar _pièce de
résistance_ of the Lamarckian school, the giraffe. He recognizes that
the chief traits in the structure of this animal are the result of
natural selection, since its efforts to reach higher branches could not
be the cause of the lengthening of the legs. But “the coadaptation of
the parts, required to make the giraffe’s structure useful, is much
greater than at first appears.” For example, the bones and the muscles
of the hind-legs have been also altered, and Spencer argues that it is
“impossible to believe” that all parts of the hind-quarters could have
been coadapted to one another, and to all parts of the fore-quarters. A
lack of coadaptation of a single muscle “would cause fatal results when
high speed had to be maintained while escaping from an enemy.”
Spencer claims that, since 1886, when he first published this argument,
nothing like an adequate response has been made; and I think he might
have added that an adequate answer is not likely to be forthcoming,
since nothing short of a demonstration of how the giraffe really evolved
is likely to be considered as sufficient. Wallace’s reply, that the
changes in question could have been brought about by natural selection,
since similar changes have been brought about by artificial selection,
is regarded as inadequate by Spencer, since it assumes a parallel which
does not exist. Nevertheless, Wallace’s reply contains, in my opinion,
the kernel of the explanation, in so far as it assumes that congenital
variation[19] may suffice to account for the origin of a form even as
bizarre as that of the giraffe. The ancon ram and the turnspit dog were
marked departures from the normal types, and yet their parts were
sufficiently coördinated for them to carry out the usual modes of
progression. It would not have been difficult, if we adopted Spencer’s
mode of arguing, to show that these new forms could not possibly have
arisen as the result of congenital variations.
Footnote 19:
Wallace assumes fluctuating variation to suffice, but in this I cannot
agree with him.
Again, it might be argued that the large, powerful dray-horse could not
have arisen through a series of variations from the ordinary horse,
because, even if variations in the right direction occurred in the
fore-quarters, it is unlikely that similar variations would occur in the
hind-quarters, etc. Yet the feat has been accomplished, and while it is
difficult to prove that the inheritance of acquired characters has not
had a hand in the process, it is improbable that this has been the case,
but rather that artificial selection of some kind of variations has been
the factor at work.
So long as the Lamarckian theory is supported by arguments like these,
it can never hope to be established with anything more than a certain
degree of probability. If it is correct, then its demonstration must
come from experiment. This brings us to a consideration of the
experimental evidence which has been supposed by some writers to give
conclusive proof of the validity of the theory.
The best direct evidence in favor of the Lamarckian argument is that
furnished by the experiments of Brown-Séquard. He found, as the result
of injury to the nervous system of guinea-pigs, that epilepsy appeared
in the adult animal, and that young born from these epileptic parents
became also epileptic. Still more important was his discovery that,
after an operation on the nerves, as a result of which certain organs,
the ear or the leg, for instance, are affected, the same affection
appears in the young born from such parents. These results of
Brown-Séquard have been vouched for by two of his assistants, and his
results in regard to the inheritance of epilepsy have been confirmed by
Obersteiner, and by Luciani on dogs. Equally important is their later
confirmation, as far as the main facts go, by Romanes.
Brown-Séquard gives the following summary of his results. I follow
Romanes’ translation in his book on “Darwin and after Darwin,” where
there is also given a careful analysis of Brown-Séquard’s results, as
well as the outcome of the experiments of Romanes himself. The summary
is as follows:—
1. “Appearance of epilepsy in animals born of parents which had been
rendered epileptic by an injury to the spinal cord.
2. Appearance of epilepsy also in animals born of parents which had been
rendered epileptic by section of the sciatic nerve.
3. A change in the shape of the ear in animals born of parents in which
such a change was the effect of a division of the cervical sympathetic
nerve.
4. Partial closure of the eyelids in animals born of parents in which
that state of the eyelids had been caused either by section of the
cervical sympathetic nerve, or the removal of the superior cervical
ganglion.
5. Exophthalmia in animals born of parents in which an injury to the
restiform body had produced that protrusion of the eyeball. This
interesting fact I have witnessed a good many times, and seen the
transmission of the morbid state of the eye continue through four
generations. In these animals modified by heredity, the two eyes
generally protruded, although in the parents usually only one showed
exophthalmia, the lesion having been made in most cases only on one of
the corpora restiformia.
6. Hæmatoma and dry gangrene of the ears in animals born of parents in
which these ear alterations had been caused by an injury to the
restiform body near the nib of the calamus.
7. Absence of two toes out of the three of the hind-leg, and sometimes
of the three, in animals whose parents had eaten up their hind-leg toes,
which had become anæsthetic from a section of the sciatic nerve alone,
or of that nerve and also of the crural. Sometimes, instead of complete
absence of the toes, only a part of one or two or three was missing in
the young, although in the parent not only the toes, but the whole foot
was absent (partly eaten off, partly destroyed by inflammation,
ulceration, or gangrene).
8. Appearance of various morbid states of the skin and hair of the neck
and face in animals born of parents having had similar alterations in
the same parts as effects of an injury to the sciatic nerve.”
Romanes, who later went over the same ground, in part under the
immediate direction of Brown-Séquard himself, has made some important
observations in regard to these results, many of which he was able to
confirm.
He did not repeat the experiment of cutting the cord, but he found that,
to produce epilepsy, it was only necessary to cut the sciatic nerve. The
“epileptiform habit” does not appear in the animal until some time after
the operation; it lasts for some weeks or months, and then disappears.
The attacks are not brought on spontaneously, but by “irritating a small
area of the skin behind the ear on the same side of the body as that on
which the sciatic nerve had been divided.” The attack lasts for only a
few minutes, and during it the animal is convulsed and unconscious.
Romanes thinks that the injury to the sciatic nerve, or to the spinal
cord, produces some sort of a change in the cerebral centres, “and that
it is this change—whatever it is, and in whatever part of the brain it
takes place—which causes the remarkable phenomena in question.”
In regard to Brown-Séquard’s statements, made in the 3d and the 4th
paragraphs, in respect to the results of the operation of cutting the
cervical sympathetic, Romanes had not confirmed the results when his
manuscript went to press; but soon afterward, after Romanes’ death, a
note was printed in _Nature_ by Dr. Hill, announcing that two
guinea-pigs from Romanes’ experiment had been born, “both of which
exhibited a well-marked droop of the upper eyelid. These guinea-pigs
were the offspring of a male and female in both of which I had produced
for Dr. Romanes, some months earlier, a droop of the left upper eyelid
by division of the left cervical sympathetic nerve. This result is a
corroboration of the series of Brown-Séquard experiments on the
inheritance of acquired characters.”
Romanes states that he also found that injury to a particular spot of
the restiform bodies is quickly followed by a protrusion of the eye on
the same side, and further, that he had “also had many cases in which
some of the progeny of parents thus affected have shown considerable
protrusion of the eyeballs of both sides, and this seemingly abnormal
protrusion has occasionally been transmitted to the next generation.
Nevertheless, I am far from satisfied that this latter fact is anything
more than an accidental coincidence.” This reservation is made on the
ground that the protrusion in the young is never so great as in the
parents, and also because there is amongst guinea-pigs a considerable
amount of individual variation in the degree of prominence of the
eyeballs. Romanes, while unwilling to deny that an “obviously abnormal
amount of protrusion, due to the operation, may be inherited in lesser
degree,” is also unwilling to affirm so important a conclusion on the
basis of these experiments alone.
In regard to Brown-Séquard’s 6th statement, Romanes found after injury
to the restiform body that hæmatoma and dry gangrene may supervene,
either several weeks after the operation, or at any subsequent time,
even many months afterward. The disease usually affects the upper parts
of both ears, and may then gradually extend downward until nearly the
whole ear is involved. “As regards the progeny of animals thus affected
in some cases, but by no means in all, a similarly morbid state of the
ears may arise apparently at any time in the life history of the
individual. But I have observed that in cases where two or more
individuals _of the same litter_ develop this diseased condition, they
usually do so at about the same time, even though this may be months
after birth, and therefore after the animals are fully grown.” Moreover,
the morbid process never extends so far in the young as it does in the
parents, and “it almost always affects the middle third of the ear.”
Several of the progeny from this first generation, which had apparently
inherited the disease, but had not themselves been directly operated
upon, showed a portion of the ear consumed apparently by the same
disease. Romanes then gives the following significant analysis of this
result. Since a different part of the ear of the progeny is affected,
and also a “very much less quantity thereof,” it might seem that the
result was due either to a mere coincidence, or to the transmission of
microbes. But he goes on to say, that he fairly well excluded both of
these possibilities, for, in the first place, he has never observed “the
very peculiar process in the ears, or in any other parts of guinea-pigs
which have neither themselves had the restiform bodies injured, nor been
born of parents thus mutilated.” In regard to microbes, Romanes tried to
infect the ears of normal guinea-pigs by first scarifying these parts,
and then rubbing them with the diseased surfaces of the ears of affected
guinea-pigs. In not a single case was the disease produced.
Romanes concludes that these “results in large measure corroborate the
statements of Brown-Séquard; and it is only fair to add that he told me
they were the results which he had himself obtained most frequently, but
that he had also met with many cases where the diseased condition of the
ears in parents affected the same parts in their progeny and also
occurred in more equal degrees.”
We come now to the remarkable conclusion given in Brown-Séquard’s 7th
statement, in regard to the absence of toes in animals whose parents had
eaten off their own hind toes and even parts of their legs. Romanes got
neuroses in the animals operated upon, and found that the toes might be
eaten off; but none of the young showed any defect in these parts.
Furthermore, Romanes repeated the same operation upon the descendants
through six successive generations, so as to produce, if possible, a
cumulative effect, but no inheritance of the mutilation was observed.
“On the other hand, Brown-Séquard informed me that he had observed this
inherited absence of toes only in about one or two per cent of cases.”
It is possible, therefore, Romanes adds, that his own experiments were
not sufficiently numerous to have obtained such cases.
In this connection I may give an account of some observations that I
made while carrying out some experiments in telegony with mice. I found
in one litter of mice that when the young came out of the nest they were
tailless. The same thing happened again when the second litter was
produced, but this time I made my observations sooner, and examined the
young mice immediately after birth. I found that the mother had bitten
off, and presumably eaten, the tails of her offspring at the time of
birth. Had I been carrying on a series of experiments to see if, when
the tails of the parents were cut off, the young inherit the defect, I
might have been led into the error of supposing that I had found such a
case in these mice. If this idiosyncrasy of the mother had reappeared in
any of her descendants, the tails might have disappeared in succeeding
generations. This perversion of the maternal instincts is not difficult
to understand, when we recall that the female mouse bites off the
navel-string of each of her young as they are born, and at the same time
eats the afterbirth. Her instinct was carried further in this case, and
the projecting tail was also removed.
Is it not possible that something of this sort took place in
Brown-Séquard’s experiment? The fact that the adults had eaten off their
own feet might be brought forward to indicate the possibility of a
perverted instinct in this case also. At least my observation shows a
possible source of error that must be guarded against in future work on
this subject.
In regard to the 8th statement of Brown-Séquard, as to various morbid
states of the skin, Romanes did not test this, because the facts which
it alleges did not seem of a sufficiently definite character.
These experiments of Brown-Séquard, and of those who have repeated them,
may appear to give a brilliant experimental confirmation of the
Lamarckian position; yet I think, if I were a Lamarckian, I should feel
very uncomfortable to have the best evidence in support of the theory
come from this source, because there are a number of facts in the
results that make them appear as though they might, after all, be the
outcome of a transmitted disease, as Weismann claims, rather than the
inheritance of an acquired character. Until we know more of the
pathology of epilepsy, it may be well not to lay too great emphasis on
these experiments. It should not be overlooked that during the long time
that the embryo is nourished in the uterus of the mother, there is ample
opportunity given for the transmission of material, or possibly even of
bacteria. If it should prove true that epilepsy is due to some substance
present in the nervous system, such substances could get there during
the uterine life of the embryo. Even if this were the case, it may be
claimed that it does not give an explanation of the local reappearance
of the disease in the offspring. But here also we must be on our guard,
for it is possible that only certain regions of the body are susceptible
to a given disease; and it has by no means been shown that the local
defect itself is inherited, but only the disease. Romanes insists that a
very special operation is necessary to bring about certain forms of
transmission.
It is well also to keep in mind the fact, that if this sort of effect is
inherited, then we must be prepared to accept as a possibility that
other kinds of injury to the parent may be transmitted to the offspring.
It would be of great disadvantage to animals if they were to inherit the
injuries that their parents have suffered in the course of their lives.
In fact, we might expect to find many plants and animals born in a
dreadful state of mutilation as a result of inheritances of this sort.
Thus, while the Lamarckians try to show that, on their principle,
characters for the good of the species may be acquired, they must also
be prepared, if they accept this kind of evidence, to grant that immense
harm may also result from its action. I do not urge this as an argument
against the theory itself, but point it out simply as one of the
consequences of the theory.
It has been shown quite recently, by Charrin, Delamare, and Moussu, that
when, after the operation of laparotomy on a pregnant rabbit or
guinea-pig, the kidney or the liver has become diseased, the offspring
sometimes show similar affections in the corresponding organs (kidney or
liver). The result is due, the authors think, to some substance set free
from the diseased kidney of the parent that affects the kidney of the
young in the uterus. By injecting into the blood of a pregnant animal
fresh extracts from the kidney of another animal, the authors believe
that the kidney of the young are also affected. It will be observed that
this transmission of an acquired character appears to be different from
that of transmission through the egg; for it is the developing, or
developed organ itself, that is acted upon. The results throw an
interesting light on the cases of epilepsy described by Brown-Séquard,
since they show that the diseased condition of the parent may be
transmitted to the later embryonic stages. May not, therefore,
Brown-Séquard’s results be also explained as due to direct transmission
from the organs of the parent to the similar organs of the young in the
uterus?
There is another series of experiments of a different sort that has been
used as an argument in favor of the Lamarckian view. These are the
results that Cunningham has obtained on young flatfish. He put the very
young fish, while still bilaterally symmetrical (in which stage the
pigment is equally developed on both sides of the body) into aquaria
lighted from below. He found that when the young fish begins to undergo
its metamorphosis, the pigment gradually disappears on one side, as it
would have done under normal conditions, _i.e._ when they are lighted
from above. If, however, the fish are kept for some time longer, lighted
from below, the pigment begins to come back again. “The first fact
proves that the disappearance of the pigment-cells from the lower side
in the metamorphosis is an hereditary character, and not a change
produced in each individual by the withdrawal of the lower side from the
action of light. On the other hand, the experiments show that the
absence of pigment-cells from the lower side throughout life is due to
the fact that light does not act upon that side, for, when it is allowed
to act, pigment-cells appear. It seems to me that the only reasonable
conclusion from these facts is, that the disappearance of pigment-cells
was originally due to the absence of light, and that the change has now
become hereditary. The pigment-cells produced by the action of light on
the lower side are in all respects similar to those normally present on
the upper side of the fish. If the disappearance of the pigment-cells
were due entirely to a variation of the germ-plasm, no external
influence could cause them to reappear, and, on the other hand, if there
were no hereditary tendency, the coloration of the lower side of the
flatfish when exposed would be rapid and complete.”[20]
Footnote 20:
_Natural Science_, October, 1893.
This evidence might be convincing were it not weakened by two or three
assumptions. In the first place, it is not shown that if the loss of
color on the lower side had been the result of the inheritance of an
acquired character that the results seen in Cunningham’s experiment
would follow as a consequence. Thus one of the starting-points of the
argument really begs the whole question. In the second place, it is
unproven that, had the loss of color of the lower side been the result
of a variation of the germ-plasm, no external influence could cause it
to reappear. In this connection there is another fact that has a bearing
on the point here raised. In some species of flatfish the right side is
turned down, and in other species the left. Occasionally an individual
is found in a right-sided species that is left-sided, and in such cases
the color is also reversed. Now, to explain this in the way suggested by
Cunningham, we should be obliged to assume that some of the ancestors
acquired the loss of pigment on one side of the body, and others on the
other side according to which side was turned down. This supposition
might be appealed to to give us an explanation of the occasional
reversal of the symmetry as a rare occurrence at the present time; but
the argument is so transparently improbable that, I believe, the
Lamarckian school would hesitate to make use of it, yet, in principle,
it is about the same as that Cunningham has followed above.
If, on the other hand, we suppose the difference in color of the two
sides to have been the result of a germ-variation, we need only suppose
that this was of such a kind that the color of the under side is only in
a latent condition, and if an external factor can cause a reaction to
take place on the light side, it is not surprising that this should call
forth the latent color patterns. The result can be given at least a
formal explanation on the theory that the original change was a
germ-variation.
We come now to the evidence derived from paleontology. A number of
evolutionists, more especially of the American school, have tried to
show that the evolution of a number of groups can best be accounted for
on the theory of the inheritance of acquired characters. A point that we
must always bear in mind is that evolution in a direct line need not
necessarily be the outcome of Lamarckian factors. Some of our leading
paleontologists, Cope, Hyatt, Scott, Osborn, have been strongly
impressed by the paleontological evidence in favor of the view that
evolution has often been in direct lines; and some, at least, of these
investigators have been led to conclude that only the Lamarckian factor
of the inheritance of acquired characters can give a sufficient
explanation of the facts. Paleontologists have been much impressed by
the fact that evolution has been along the lines which we might imagine
that it would follow if the effects of use and of disuse are inherited.
There is, however, no proof that this is the case, although there are a
number of instances to which this mode of explanation appears to give
the readiest solution. But, as has been said before, it is not this kind
of evidence that the theory is in need of, since Lamarck himself gave an
ample supply of illustrations. What we need is clear evidence that this
sort of inheritance is possible, and, from the very nature of the case,
it is just this evidence that fossil remains can never supply.
The same criticism may be made of the work of Ryder, Packard, Dali,
Jackson, Eimer, Cunningham, Semper, De Varigny, and others of the
Lamarckian school. Despite the large number of cases that they have
collected, which appear to them to be most easily explained on the
assumption of the inheritance of acquired characters, the proof that
such inheritance is possible is not forthcoming. Why not then spend a
small part of the energy, that has been used to expound the theory, in
demonstrating that such a thing is really possible? One of the chief
virtues of the Lamarckian theory is that it is capable of experimental
verification or contradiction, and who can be expected to furnish such
proof if not the Neo-Lamarckians?
We may fairly sum up our position in regard to the theory of the
inheritance of acquired characters in the verdict of “not proven.” I am
not sure that we should not be justified at present in claiming that the
theory is unnecessary and even improbable.
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