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 [2], "that few naturalists nowadays,
in describing and proposing a name for what they call 'a new species,'
use that term to signify what was meant by it twenty or thirty years
ago; that is, an originally distinct creation, maintaining its primitive
distinction by obstructive generative peculiarities. The proposer of the
new species now intends to state no more than he actually knows; as, for
example, that the differences on which he founds the specific character
are constant in individuals of both sexes, so far as observation has
reached; and that they are not due to domestication or to artificially
superinduced external circumstances, or to any outward influence within
his cognizance; that the species is wild, or is such a