more than a kind of animal or plant, which is distinctly definable from all
others, by certain constant, and not merely sexual, morphological
peculiarities. Thus horses form a species, because the group of animals to
which that name is applied is distinguished from all others in the world by
the following constantly associated characters. They have--1, A vertebral
column; 2, Mammae; 3, A placental embryo; 4, Four legs; 5, A single
well-developed toe in each foot provided with a hoof; 6, A bushy tail; and
7, Callosities on the inner sides of both the fore and the hind legs. The
asses, again, form a distinct species, because, with the same characters,
as far as the fifth in the above list, all asses have tufted tails, and
have callosities only on the inner side of the fore-legs. If animals were
discovered having the general characters of the horse, but sometimes with
callosities only on the fore-legs, and more or less tufted tails; or
animals having the general characters of the ass, but with more or less
bushy tails, and sometimes with callosities on both pairs of legs, besides
being intermediate in other respects--the two species would have to be
merged into one. They could no longer be regarded as morphologically
distinct species, for they would not be distinctly definable one from the
other.

However bare and simple this definition of species may appear to be, we
confidently appeal to all practical naturalists, whether zoologists,
botanists, or palaeontologists, to say if, in the vast majority of cases,
they know, or mean to affirm, anything more of the group of animals or
plants they so denominate than what has just been stated. Even the most
decided advocates of the received doctrines respecting species admit this.

"I apprehend," says Professor Owen, [Footnote: "On the Osteology of the
Chimpanzees and Orangs"; _Transactions of the Zoological Society_,
1858.] "that few naturalists nowadays, in describing and proposing a name