limits of species, and, what is more, that when a variety has come into
existence in nature, there are natural causes and conditions, which are
amply competent to play the part of a selective breeder; and although
that is not quite the evidence that one would like to have--though it
is not direct testimony--yet it is exceeding good and exceedingly
powerful evidence in its way.

As to the first point, of varieties existing among natural species, I
might appeal to the universal experience of every naturalist, and of
any person who has ever turned any attention at all to the
characteristics of plants and animals in a state of nature; but I may as
well take a few definite cases, and I will begin with Man himself.

I am one of those who believe that, at present, there is no evidence
whatever for saying, that mankind sprang originally from any more than
a single pair; I must say, that I cannot see any good ground whatever,
or even any tenable sort of evidence, for believing that there is more
than one species of Man. Nevertheless, as you know, just as there are
numbers of varieties in animals, so there are remarkable varieties of
men. I speak not merely of those broad and distinct variations which
you see at a glance. Everybody, of course, knows the difference
between a Negro and a white man, and can tell a Chinaman from an
Englishman. They each have peculiar characteristics of colour and
physiognomy; but you must recollect that the characters of these races
go very far deeper--they extend to the bony structure, and to the
characters of that most important of all organs to us--the brain; so
that, among men belonging to different races, or even within the same
race, one man shall have a brain a third, or half, or even seventy per
cent. bigger than another; and if you take the whole range of human
brains, you will find a variation in some cases of a hundred per cent.