Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

310 THE ACTION OF VATURAL
the selected portion of each succeeding generation will
therefore be stronger, swifter, and more thickly furred
than the last; and if this process goes on for thousands
of generations, our animal will have again become
thoroughly in harmony with the new conditions in
which it is placed. But it will now be a different
creature. It will be not only swifter and stronger,
and more furry, it will also probably have changed in
colour, in form, perhaps have acquired a longer tail,
or differently shaped ears; for it is an ascertained
fact, that when one part of an animal is modified,
some other parts almost always change, as it were in
sympathy with it. Mr. Darwin calls this " correlation
of growth," and gives as instances, that hairless dogs
have imperfect teeth; white cats, when blue-eyed, are
deaf; small feet accompany short beaks in pigeons;
and other equally interesting cases.
Grant, therefore, the premises: 1st. That peculiarities of every kind are more or less hereditary. 2nd.
That the offspring of every animal vary more or less
in all parts of their organization. 3rd. That the
universe in which these animals live, is not absolutely
invariable; —none of which propositions can be denied;
and then consider, that the animals in any country
(those at least which are not dying out) must at each
successive period be brought into harmony with the
surrounding conditions; and we have all the elements
for a change of form and structure in the animals,
keeping exact pace with changes of whatever nature
in the surrounding universe. Such changes must be