an organ may, under certain circumstances, be beneficial to the
species. For example, he first proved in the instance of Madeira, that
the loss of wings may be of advantage to many beetles inhabiting
oceanic islands. The individuals with imperfectly developed or
atrophied wings have an advantage, because they are not carried out to
sea by the frequent winds. The small eyes, buried in fur, possessed by
moles and other subterranean mammals, can be similarly explained by
means of natural selection. So also, the complete disappearance of the
limbs of snakes is evidently a real advantage to animals which creep
through narrow holes and clefts; and the degeneration of the wings in
the ostrich and penguin is, in part, explicable as a favourable
modification of the organ of flight into an organ for striking air or
water respectively.
But when the degeneration of disused organs confers no benefits upon
the individual, the explanation becomes less simple. Thus we find that
the eyes of animals which inhabit dark caves (such as insects, crabs,
fish, Amphibia, etc.) have undergone degeneration; yet this can hardly
be of direct advantage to the animals, for they could live quite as
well in the dark with well-developed eyes. But we are here brought into
contact with a very important aspect of natural selection, viz. the
power of conservation exerted by it. Not only does the survival of the
fittest select the best, but it also maintains it[45]. The struggle for
existence does not cease with the foundation of a new specific type, or
with some perfect adaptation to the external or internal conditions of
life, but it becomes, on the contrary, even more severe, so that the
most minute differences of structure determine the issue between life
and death.
The sharpest sight possessed by birds is found in birds of prey, but if
one of them entered the world with eyes rather below the average in
this respect, it could not, in the long run, escape death from hunger,
because it would always be at a disadvantage as compared with others.
Hence the sharp sight of these birds is maintained by means of the
continued operation of natural selection, by which the individuals with
the weakest sight are being continually exterminated. But all this
would be changed at once, if a bird of prey of a certain species were
compelled to live in absolute darkness. The quality of the eyes would
then be immaterial, for it could make no difference to the existence of
the individual, or the maintenance of the species. The sharp sight
might, perhaps, be transmitted through numerous generations; but when
weaker eyes arose from time to time, these would also be transmitted,
for even very short-sighted or imperfect eyes would bring no
disadvantage to their owner. Hence, by continual crossing between
individuals with the most varied degrees of perfection in this respect,
the average of perfection would gradually decline from the point
attained before the species lived in the dark.
We do not at present know of any bird living in perfect darkness, and
it is improbable that such a bird will ever be found; but we are
acquainted with blind fish and Amphibia, and among these the eyes are
present it is true, but they are small and hidden under the skin. I
think it is difficult to reconcile the facts of the case with the
ordinary theory that the eyes of these animals have simply degenerated
through disuse. If disuse were able to bring about the complete atrophy
of an organ, it follows that every trace of it would be effaced. We
know that, as a matter of fact, the olfactory organ of the frog
completely degenerates when the olfactory nerve is divided; and that
great degeneration of the eye may be brought about by the artificial
destruction of the optic centre in the brain. Since, therefore, the
effects of disuse are so striking in a single life, we should certainly
expect, if such effects can be transmitted, that all traces of an eye
would soon disappear from a species which lives in the dark.
The caverns in Carniola and Carinthia, in which the blind _Proteus_ and
so many other blind animals live, belong geologically to the Jurassic
formation; and although we do not exactly know when for example the
_Proteus_ first entered them, the low organization of this amphibian
certainly indicates that it has been sheltered there for a very long
period of time, and that thousands of generations of this species have
succeeded one another in the caves.
Hence there is no reason to wonder at the extent to which the
degeneration of the eye has been already carried in the _Proteus_; even
if we assume that it is merely due to the cessation of the conserving
influence of natural selection.
But it is unnecessary to depend upon this assumption alone, for when a
useless organ degenerates, there are also other factors which demand
consideration, namely, the higher development of other organs which
compensate for the loss of the degenerating structure, or the increase
in size of adjacent parts. If these newer developments are of advantage
to the species, they finally come to take the place of the organ which
natural selection has failed to preserve at its point of highest
perfection.
In the first place, a certain form of correlation, which Roux[46] calls
‘the struggle of the parts in the organism,’ plays a most important
part. Cases of atrophy, following disuse, appear to be always attended
by a corresponding increase of other organs: blind animals always
possess very strongly developed organs of touch, hearing, and smell,
and the degeneration of the wing-muscles of the ostrich is accompanied
by a great increase in the strength of the muscles of the leg. If the
average amount of food which an animal can assimilate every day remains
constant for a considerable time, it follows that a strong influx
towards one organ must be accompanied by a drain upon others, and this
tendency will increase, from generation to generation, in proportion to
the development of the growing organ, which is favoured by natural
selection in its increased blood-supply, etc.; while the operation of
natural selection has also determined the organ which can bear a
corresponding loss without detriment to the organism as a whole.
Without the operation of natural selection upon different individuals,
the struggle between the organs of a single individual would be unable
to encourage a predisposition in the germ towards the degeneration or
non-development of a useless organ, and it could only limit and degrade
the development of an organ in the lifetime of the individual. If,
therefore, acquired characters are not transmitted, the disposition to
develope such an organ would be present in the same degree in each
successive generation, although the realization would be less perfect.
The complete disappearance of a rudimentary organ can only take place
by the operation of natural selection; this principle will lead to its
elimination, inasmuch as the disappearing structure takes the place and
the nutriment of other useful and important organs. Hence the process
of natural selection tends to entirely remove the former. The
predisposition towards a weaker development of the organ is thus
advantageous, and there is every reason for the belief that the
advantages would continue to be gained, and that therefore the
processes of natural selection would remain in operation, until the
germ had entirely lost all tendency towards the development of the
organ in question. The extreme slowness with which this process takes
place, and the extraordinary persistence of rudimentary organs, at any
rate in the embryo, together with their gradual but finally complete
disappearance, can be clearly seen in the limbs of certain vertebrates
and arthropods. The blind-worms have no limbs, but a rudimentary
shoulder-girdle is present close under the skin, and the interesting
fact has been quite recently established[47] that the fore-limbs are
present in the embryo in the form of short stumps, which entirely
disappear at a later stage. In most snakes all traces of limbs have
been lost in the adult, but we do not yet know for certain whether they
are also wanting in the embryo. I might further mention the very
different stages of degeneration witnessed in the limbs of various
salamanders; and the anterior limbs of _Hesperornis_—the remarkable
toothed bird from the cretaceous rocks—which, according to Marsh[48],
consists only of a very thin and relatively small humerus, which was
probably concealed beneath the skin. The water-fleas (_Daphnidae_)
possess in the embryonic state three complete and almost equal pairs of
jaws, but two of these entirely disappear, and do not develope into
jaws in any species. In the same way, the embryo of the maggot-like
legless larva of bees and wasps possesses three pairs of ancestral
limbs.
There are, however, cases in which, apparently, acquired variations of
characters are transmitted without natural selection playing any active
part in the change. Such a case is afforded by the short-sightedness so
common in civilized nations.
This affection is certainly hereditary in some cases, and it may well
have been explained as an example of the transmission of acquired
changes. It has been argued that acquired short-sightedness can be in a
slight degree transmitted, and that each successive generation has
developed a further degree of the disease by habitually holding books
etc. close to the eyes, so that the inborn predisposition to
short-sightedness is continually accumulating.
But we must remember that variations in the refraction of the human eye
have been for a long time independent of the preserving control of
natural selection. In the struggle for existence, a blind man would
certainly disappear before those endowed with sight, but myopia does
not prevent any one from gaining a living.
A short-sighted lynx, hawk, or gazelle, or even a short-sighted Indian,
would be eliminated by natural selection, but a short-sighted European
of the higher class finds no difficulty in earning his bread.
Those fluctuations on either side of the average which we call myopia
and hypermetropia, occur in the same manner, and are due to the same
causes, as those which operate in producing degeneration in the eyes of
cave-dwelling animals. If, therefore, we not infrequently meet with
families in which myopia is hereditary, such results may be attributed
to the transmission of an accidental disposition on the part of the
germ, instead of to the transmission of acquired short-sightedness. A
very large proportion of short-sighted people do not owe their
affliction to inheritance at all, but have acquired it for themselves;
for there is no doubt that a normal eye may be rendered myopic in the
course of a life-time by continually looking at objects from a very
short distance, even when no hereditary predisposition towards the
disease can be shown to exist. Such a change would of course appear
more readily if there was also a corresponding predisposition on the
part of the eye. But I should not explain this widely spread
predisposition towards myopia as due to the transmission of acquired
short-sightedness, but to the greater variability of the eye, which
necessarily results from the cessation of the controlling influence of
natural selection.
This suspension of the preserving influence of natural selection may be
termed _Panmixia_, for all individuals can reproduce themselves and
thus stamp their characters upon the species, and not only those which
are in all respects, or in respect to some single organ, the fittest.
In my opinion, the greater number of those variations which are usually
attributed to the direct influence of external conditions of life, are
to be ascribed to panmixia. For example, the great variability of most
domesticated animals essentially depends upon this principle.
A goose or a duck must possess strong powers of flight in the natural
state, but such powers are no longer necessary for obtaining food when
it is brought into the poultry-yard, so that a rigid selection of
individuals with well-developed wings, at once ceases among its
descendants. Hence in the course of generations, a deterioration of the
organs of flight must necessarily ensue, and the other members and
organs of the bird will be similarly affected.
This example very clearly indicates that the degeneration of an organ
does not depend upon its disuse; for although our domestic poultry very
rarely make use of their wings, the muscles of flight have not
disappeared, and, at any rate in the goose, do not seem to have
undergone any marked degeneration.
The numerous and exact observations conducted by Darwin upon the weight
and measurement of the bones in domestic fowls, seem to me to possess a
significance beyond that which he attributed to them.
If the weight of the wing-bones of the domestic duck bears a smaller
proportion to the weight of the leg-bones than in the wild duck, and
if, as Darwin rightly assumes, this depends not only upon the
diminution of the wings, but also upon the increase of the legs, it by
no means follows that this latter increase in organs which are now more
frequently used, is dependent upon hereditary influences alone.
It is quite possible that it depends, on the one hand, upon the
suspension of natural selection, or panmixia (and these effects would
be transmitted), and on the other hand upon the direct influence of
increased use during the course of a single life. We do not yet know
with any accuracy, the amount of change which may be produced by
increased use in the course of a single life. If it is desired to prove
that use and disuse produce hereditary effects without the assistance
of natural selection, it will be necessary to domesticate wild animals
(for example the wild duck) and preserve all their descendants, thus
excluding the operation of natural selection. If then all individuals
of the second, third, fourth and later generations of these tame ducks
possess identical variations, which increase from generation to
generation, and if the nature of these changes proves that they must
have been due to the effect of use or disuse, then perhaps the
transmission of such effects may be admitted; but it must always be
remembered that domestication itself influences the organism,—not only
directly, but also indirectly, by the increase of variability as a
result of the suspension of natural selection. Such experiments have
not yet been carried out in sufficient detail[49].
It is usually considered that the origin and variation of instincts are
also dependent upon the exercise of certain groups of muscles and
nerves during a single life-time; and that the gradual improvement
which is thus caused by practice, is accumulated by hereditary
transmission. I believe that this is an entirely erroneous view, and I
hold that all instinct is entirely due to the operation of natural
selection, and has its foundation, not upon inherited experiences, but
upon the variations of the germ.
Why, for instance, should not the instinct to fly from enemies have
arisen by the survival of those individuals which are naturally timid
and easily startled, together with the extermination of those which are
unwary? It may be urged in opposition to this explanation that the
birds of uninhabited islands which are not at first shy of man, acquire
in a few generations an instinctive dread of him, an instinct which
cannot have arisen in so short a time by means of natural selection.
But, in this case are we really dealing with the origin of a new
instinct, or only with the addition of one new perception
(‘Wahrnehmung,’ Schneider)[50], of the same kind as those which incite
to the instinct of flight—an instinct which had been previously
developed in past ages but had never been called forth by man? Again,
has any one ascertained whether the young birds of the second or third
generation are frightened by man? May it not be that the experience of
a single life-time plays a great part in the origin of the habit? For
my part, I am inclined to believe that the habit of flying from man is
developed in the first generation which encounters him as a foe[51]. We
see how wary and cautious a flock of birds become as soon as a few
shots have been fired at them, and yet shortly before this occurrence
they were perhaps playing carelessly close to the sportsmen.
Intelligence plays a considerable part in the life of birds, and it by
no means follows that the transmission of individual habits explains
the above-mentioned phenomena. The long-continued operation of natural
selection may very well have been necessary before the perception of
man could awake the instinct to flee in young, inexperienced birds.
Unfortunately the observations upon these points are far too indefinite
to enable us to draw conclusions.
There is again the frequently-quoted instance of the young pointer,
‘which, untrained, and without any example which might have been
imitated, pointed at a lizard in a subtropical jungle, just as many of
its forefathers had pointed at partridges on the plain of St. Denis,’
and which, without knowing the effect of a shot, sprang forward
barking, at the first discharge, to bring in the game. This conduct
must not be attributed to the inheritance of any mental picture, such
as the effect of a shot, but to the inheritance of a certain reflex
mechanism. The young pointer does not spring forward at the shot
because he has inherited from his forefathers a certain association of
ideas,—shot and game,—but because he has inherited a reflex mechanism,
which impels him to start forward on hearing a report. We cannot yet
determine without more experiments how such an impulse due to
perception (‘Wahrnehmungstrieb,’ Schneider) has arisen; but, in my
opinion, it is almost inconceivable that artificial breeding has had
nothing to do with it; and that we are here concerned—not with the
inheritance of the effects of training—but with some predisposition on
the part of the germ, which has been increased by artificial selection.
The necessity for extreme caution in appealing to the supposed
hereditary effects of use, is well shown in the case of those numerous
instincts, which only come into play once in a lifetime, and which do
not therefore admit of improvement by practice. The queen-bee takes her
nuptial flight only once, and yet how many and complex are the
instincts and the reflex mechanisms which come into play on that
occasion. Again, in many insects the deposition of eggs occurs but once
in a life-time, and yet such insects always fulfil the necessary
conditions with unfailing accuracy, either simply dropping the eggs
into water, or carefully fixing them on the surface of the earth
beneath some stone, or laying them on a particular part of a certain
species of plant; and in all these cases the most complicated actions
are performed. It is indeed astonishing to watch one of the _Cynipidae_
(_Rhodites rosae_) depositing her eggs in the tissue of a young bud.
She first carefully examines the bud on all sides, and feels it with
her legs and antennae. Then she slowly inserts her long ovipositor
between the closely-rolled leaves of the bud, but if it does not reach
exactly the right spot, she will withdraw and re-insert it many times,
until at length, when the proper place has been found, she will slowly
bore deep into the very centre of the bud, so that the egg will reach
the exact spot, where the necessary conditions for its development
alone exist.
But each _Cynips_ lays eggs many times, and it may be argued that
practice may have led to improvement in this case; we cannot however,
as a matter of fact, expect much improvement in a process which is
repeated, perhaps a dozen times, at short intervals of time, and which
is of such an excessively complex nature.
It is the same with the deposition of eggs in most insects. How can
practice have had any influence upon the origin of the instinct which
leads one of our butterflies—(_Vanessa levana_)—to lay its green eggs
in single file, as columns, which project freely from the stem or leaf,
so that protection is gained by their close resemblance to the
flower-buds of the stinging-nettle, which forms the food-plant of their
caterpillars?
Of course the butterfly is not aware of the advantage which follows
from such a proceeding; intelligence has no part in the process. The
entire operation depends upon certain inherent anatomical and
physiological arrangements:—on the structure of the ovary and oviducts,
on the simultaneous ripening of a certain number of eggs, and on
certain very complex reflex mechanisms which compel the butterfly to
lay its eggs on certain parts of certain plants. Schneider is certainly
right when he maintains that this mechanism is released by a sensation,
arising from the perception (whether by sight or smell, or both
together) of the particular plant or part of the plant upon which the
eggs are to be laid[52]. At any rate, we cannot, in such cases, appeal
to the effects of constant use and the transmission of acquired
characters, as an explanation; and the origin of the impulse can only
be understood as a result of the process of natural selection.
The protective cocoons by which the pupae of many insects are
surrounded also belong to the same category, and improvement by
practice is entirely out of the question, for they are only constructed
once in the course of a life-time. And yet these cocoons are often
remarkably complex: think, for instance, of the cocoon spun by the
caterpillar of the emperor moth (_Saturnia carpini_), which is so tough
that it can hardly be torn, and which the moth would be unable to
leave, if an opening were not provided for the purpose; while, on the
other hand, the pupa would not be defended against enemies if the
opening were not furnished with a circle of pointed bristles,
converging outwards, on the principle of the lobster pot, so that the
moth can easily emerge, although no enemy can enter. The impulse which
leads to the production of such a structure can only have arisen by the
operation of natural selection—not, of course, during the history of a
single species, but during the development of numerous, consecutive
species—by gradual and unceasing improvements in the initial stages of
cocoon-building. A number of species exists at the present day, of
which the cocoons can be arranged in a complete series, becoming
gradually less and less complex, from that described above, down to a
loosely-constructed, spherical case in which the pupa is contained.
The cocoon spun by the larva of _Saturnia carpini_ differs but little
in complexity from the web of the spider, and if the former is
constructed without assistance from the experience of the single
individual—and this must certainly be admitted—it follows that the
latter may be also built without the aid of experience, while there is
neither reason nor necessity for appealing to the entirely unproved
transmission of acquired skill in order to explain this and a thousand
other operations.
It may be objected that, in man, in addition to the instincts inherent
in every individual, special individual predispositions are also found,
of such a nature that it is impossible that they can have arisen by
individual variations of the germ. On the other hand, these
predispositions—which we call talents—cannot have arisen through
natural selection, because life is in no way dependent upon their
presence, and there seems to be no way of explaining their origin
except by an assumption of the summation of the skill attained by
exercise in the course of each single life. In this case, therefore, we
seem at first sight to be compelled to accept the transmission of
acquired characters.