Use-inheritance would crudely and indiscriminately proportion parts to
actual work done--or rather to the varying _nourishment and growth_
resulting from a multiplicity of causes--and this in its various details
would often conflict most seriously with the real necessities of the
case, such as occasional passive strength, or appropriate shape,
lightness and general adaptation. If its accumulated effects were not
corrected by natural or sexual selection, horns and antlers would
disappear in favour of enlarged hoofs. The elephant's tusks would become
smaller than its teeth. Men would have callosities for sitting on, like
certain monkeys, and huge corns or hoofs for walking on. Bones would
often be modified disastrously. Thus the condyle of the human jaw would
become larger than the body of the jaw, because as the fulcrum of the
lever it receives more pressure. Some organs (like the heart, which is
always at work) would become inconveniently or unnecessarily large.
Other absolutely indispensable organs, which are comparatively passive
or are very seldom used, would dwindle until their weakness caused the
ruin of the individual or the extinction of the species. In eliminating
various evil results of use-inheritance, natural selection would be
eliminating use-inheritance itself. The displacement of Lamarck's theory
by Darwin's shows that the effects of use-inheritance often differ from
those required by natural selection; and it is clear that the latter
factor must at least have reduced use-inheritance to the very minor
position of comparative feebleness and harmlessness assigned to it by
Darwin.
Use-inheritance would be ruinous through causing unequal variation in
co-operative parts--of which Mr. Spencer may accept his own instances of
the jaws and teeth, and the cave-crab's lost eyes and persistent
eye-stalks, as typical examples. That the variation would be unequal
seems almost self-evident from the varying rapidity and extent of the
effects of use and disuse on different tissues and on different parts of
the general structure. The optic nerve may atrophy in a few months from
disuse consequent on the loss of the eye. Some of the bones of the
rudimentary hind legs of the whale are still in existence after disuse
for an enormous period. Evidently use-inheritance could not equally
modify the turtle and its shell, or the brain and its skull; and in
minor matters there would be the same incongruity of effect. Thus, if
the molar teeth lengthened from extra use the incisors could not meet.
Unequal and indiscriminate variation would throw the machinery of the
organism out of gear in innumerable ways.
Use-inheritance would perpetuate various evils. We are taught, for
instance, that it perpetuates short-sight, inferior senses, epilepsy,
insanity, nervous disorders, and so forth. It would apparently transmit
the evil effects of over-exertion, disuse, hardship, exposure, disease
and accident, as well as the defects of age or immaturity.
Would it not be better on the whole if each individual took a fresh
start as far as possible on the advantageous typical lines laid down by
natural selection? Through the long stages of evolution from primaeval
protoplasm upwards, such species as were least affected by
use-inheritance would be most free to develop necessary but seldom-used
organs, protective coverings such as shells or skulls, and natural
weapons, defences, ornaments, special adaptations, and so forth; and
this would be an advantage--for survival would obviously depend on the
_importance_ of a structure or faculty in deciding the struggle for
existence and reproduction, and not on the total amount of its using or
nourishment. If natural selection had on the whole favoured this
officious ally and frequent enemy, surely we should find better evidence
of its existence.
Without laying undue stress upon the evil effects of use-inheritance, a
careful examination of them in detail may at least serve to
counter-balance the optimistic _a priori_ arguments for belief in that
plausible but unproven factor of evolution.
The benefits derivable from use-inheritance are largely illusory. The
effects of _use_, indeed, are generally beneficial up to a certain
point; for natural selection has sanctioned or evolved organs which
possess the property or potentiality of developing to the right extent
under the stimulus of use or nourishment. But use-_inheritance_ would
cumulatively alter this individual adaptability, and would tend to fix
the size of organs by the average amount of ancestral use or disuse
rather than by the actual requirements of the individual. Of course
under changed conditions involving increased or lessened use of parts it
might become advantageous; but even here it may prove a decided
hindrance to adaptive evolution in some respects as well as an
unnecessary aid in others. Thus in the case of animals becoming heavier,
or walking more, it would _lengthen_ the legs although natural selection
might require them to be shortened. In the Aylesbury duck and the Call
duck, if use-inheritance has increased the dimensions of the bones and
tendons of the leg, natural selection has had to counteract this
increase so far as length is concerned, and to effect 8 per cent. of
shortening besides. If use-inheritance thickens bones without
proportionally lengthening them, it would hinder rather than help the
evolution of such structures as the long light wings of birds, or the
long legs and neck of the giraffe or crane.