der the changed
conditions of domestication. And if there be any variability under nature,
it would be an unaccountable fact if natural selection had not come into
play. It has often been asserted, but the assertion is quite incapable of
proof, that the amount of variation under nature is a strictly limited
quantity. Man, though acting on external characters alone and often
capriciously, can produce within a short period a great result by adding up
mere individual differences in his domestic productions; and every one
admits that there are at least individual differences in species under
{469} nature. But, besides such differences, all naturalists have admitted
the existence of varieties, which they think sufficiently distinct to be
worthy of record in systematic works. No one can draw any clear distinction
between individual differences and slight varieties; or between more
plainly marked varieties and sub-species, and species. Let it be observed
how naturalists differ in the rank which they assign to the many
representative forms in Europe and North America.

If then we have under nature variability and a powerful agent always ready
to act and select, why should we doubt that variations in any way useful to
beings, under their excessively complex relations of life, would be
preserved, accumulated, and inherited? Why, if man can by patience select
variations most useful to himself, should nature fail in selecting
variations useful, under changing conditions of life, to her living
products? What limit can be put to this power, acting during long ages and
rigidly scrutinising the whole constitution, structure, and habits of each
creature,--favouring the good and rejecting the bad? I can see no limit to
this power, in slowly and beautifully adapting each form to the most
complex relations of life. The theory of natural selection, even if we
looked no further than this, seems to me to be in itself probable. I have
already recapitulated, as fairly as I could, the opposed difficulties and
objections: now let us turn to the special facts and arguments in favour of
the theory.

On the view that species are only strongly marked and permanent varieties,
and that each species first existed as a variety, we can see why it is that
no line of demarcation can be drawn between species, commonly supposed to
have been produced by special acts of creation, and varieties which are
acknowledged to have been produced by secondary laws. On t