be exposed to the mutual action of a different set of inhabitants,
which I believe to be far more important to the life of each being than
mere climate. Considering the infinitely various methods which living
beings follow to obtain food by struggling with other organisms, to
escape danger at various times of life, to have their eggs or seeds
disseminated, &c. &c., I cannot doubt that during millions of
generations individuals of a species will be occasionally born with some
slight variation, profitable to some part of their economy. Such
individuals will have a better chance of surviving, and of propagating
their new and slightly different structure; and the modification may be
slowly increased by the accumulative action of natural selection to any
profitable extent. The variety thus formed will either coexist with, or,
more commonly, will exterminate its parent form. An organic being, like
the woodpecker or misseltoe, may thus come to be adapted to a score of
contingences--natural selection accumulating those slight variations in
all parts of its structure, which are in any way useful to it during any
part of its life.

5. Multiform difficulties will occur to every one, with respect to this
theory. Many can, I think, be satisfactorily answered. _Natura non facit
saltum_ answers some of the most obvious. The slowness of the change,
and only a very few individuals undergoing change at any one time,
answers others. The extreme imperfection of our geological records
answers others.

6. Another principle, which may be called the principle of divergence,
plays, I believe, an important part in the origin of species. The same
spot will support more life if occupied by very diverse forms. We see
this in the many generic forms in a square yard of turf, and in the
plants or insects on any little uniform islet, belonging almost
invariably to as many genera and families as species. We can understand
the meaning of this fact amongst the higher animals, whose habits we
understand. We know that it has been experimentally shown that a plot of
land will yield a greater weight if sown with several species and genera
of grasses, than if sown with only two or three species. Now, every
organic being, by propagating so rapidly, may be said to be striving its
utmost to increase in numbers. So it will be with the offspring of any
species after it has become diversified into varieties, or subspecies,
or true species. And it follows, I think,