lustrate
the inheritance of functionally-produced modifications, he would have
seen that his inference is far from correct. I have instanced the
decrease of the jaw among civilized men as a change of structure which
cannot have been produced by the inheritance of spontaneous, or
fortuitous, variations. That changes of structure arising from such
variations may be maintained and increased in successive generations, it
is needful that the individuals in whom they occur shall derive from
them advantages in the struggle for existence--advantages, too,
sufficiently great to aid their survival and multiplication in
considerable degrees. But a decrease of jaw reducing its weight by even
an ounce (which would be a large variation), cannot, by either smaller
weight carried or smaller nutrition required, have appreciably
advantaged any person in the battle of life. Even supposing such
diminution of jaw to be beneficial (and in the resulting decay of teeth
it entails great evils), the benefit can hardly have been such as to
increase the relative multiplication of families in which it occurred
generation after generation. Unless it has done this, however, decreased
size of the jaw cannot have been produced by the natural selection of
favourable variations. How can it then have been produced? Only by
decreased function--by the habitual use of soft food, joined, probably,
with disuse of the teeth as tools. And now mark that this cause operates
on all members of a society which falls into civilized habits.
Generation after generation this decreased function changes its
component families simultaneously. Natural selection does not cover the
case at all--has nothing to do with it. And the like happens in
multitudinous other cases. Every species spreading into a new habitat,
coming in contact with new food, exposed to a different temperature, to
a drier or moister air, to a more irregular surface, to a new soil, &c.,
&c., has its members one and all subject to various changed actions,
which influence its muscular, vascular, respiratory, digestive,
and other systems of organs. If there is inheritance of
functionally-produced modifications, then all its members will transmit
the structural alterations wrought in them, and the species will change
as a whole without the supplanting of some stocks by others. Doubtless
in respect of certain changes natural selection will co-operate. If the
species, being a predacious one, is brought, by migrati