uld sometimes run into abnormal structures were it not
for the fact that sexual reproduction constantly tends to reduce them.
In an open country where animals and plants interbreed freely, it will
commonly happen that individuals with certain peculiarities will mate
with others without such peculiarities, and the offspring will therefore
inherit the peculiarity not in increased degree but in decreased degree.
This constant interbreeding of individuals will tend to prevent the
formation of many modifications in the machine which become started by
variations. Now plainly if some such individuals, with a peculiar
variation, should migrate into a new territory or become isolated from
their relatives which do not have similar variations, these individuals
will be obliged to breed with each other. The result will be that the
next generation, arising thus from two parents each of which shows the
same variation, will show it also in equal or increased degree.
Migrations and isolations will thus tend to _fix_ in the machine
variations which sexual union or other influences inaugurate. Now in the
history of the earth's surface there have been many changes which tend
to bring about such migration and isolations, and this factor has
doubtless played a more or less important part in the building of the
machines. How great a part we cannot say, nor is it necessary for our
purpose to decide; for in all these cases the machine building has only
been the result of the hereditary transmission of congenital variation
under certain peculiar conditions. The fundamental process is the same
as already considered, only the details of its working being in
question.

==Direct Influence of the Environment.==--Under this head we have a
subject of great importance. It is an undoubted fact that the
environment has a very decided effect upon the machine. These direct
effects of the environment are very positive and in great variety. The
tropical sun darkens the human skin; cold climate stunts the growth of
plants; lack of food dwarfs all animals and plants, and hundreds of
other similar examples could be selected. Another class of similar
influences are those produced by _use_ and _disuse_. Beyond question the
use of an organ tends to increase its size, and disuse to decrease it.
Combats of animals with each other tend to increase their strength,
flight from enemies their running powers, etc.

Now all these effects are direct modifications of the ma