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). These
also produce their like and never produce each other or intermingle, and
they were therefore each supposed to have sprung from a single
individual whose "origin" was unknown.
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