work, and the change which it has effected not only in natural
history but in many other sciences, it is necessary to form a clear
conception of the meaning of the term "species," to know what was the
general belief regarding them at the time when Mr. Darwin's book first
appeared, and to understand what he meant, and what was generally meant,
by discovering their "origin." It is for want of this preliminary
knowledge that the majority of educated persons who are not naturalists
are so ready to accept the innumerable objections, criticisms, and
difficulties of its opponents as proofs that the Darwinian theory is
unsound, while it also renders them unable to appreciate, or even to
comprehend, the vast change which that theory has effected in the whole
mass of thought and opinion on the great question of evolution.

The term "species" was thus defined by the celebrated botanist De
Candolle: "A species is a collection of all the individuals which
resemble each other more than they resemble anything else, which can by
mutual fecundation produce fertile individuals, and which reproduce
themselves by generation, in such a manner that we may from analogy
suppose them all to have sprung from one single individual." And the
zoologist Swainson gives a somewhat similar definition: "A species, in
the usual acceptation of the term, is an animal which, in a state of
nature, is distinguished by certain peculiarities of form, size, colour,
or other circumstances, from another animal. It propagates, 'after its
kind,' individuals perfectly resembling the parent; its peculiarities,
therefore, are permanent."[1]

To illustrate these definitions we will take two common English birds,
the rook (Corvus frugilegus) and the crow (Corvus corone). These are
distinct _species_, because, in the first place, they always differ from
each other in certain slight peculiarities of structure, form, and
habits, and, in the second place, because rooks always produce rooks,
and crows produce crows, and they do not interbreed. It was therefore
concluded that all the rooks in the world had descended from a single
pair of rooks, and the crows in like manner from a single pair of crows,
while it was considered impossible that crows could have descended from
rooks or _vice versa_. The "origin" of the first pair of each kind was a
mystery. Similar remarks may be applied to our two common plants, the
sweet violet (Viola odorata) and the dog violet (Viola canina).