eval
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.

VARIED EFFECTS OF USE AND DISUSE.

The changes which we somewhat roughly and empirically group together as
the effects of "use and disuse" are of