"——non tamen irritum
Diffinget, infectumque reddet
Quod fugiens semel hora vexit."
On the other hand the proof of the original descent is an inference rather than a fact either ascertained or capable of being so.
XXIV.
The definition of the term species upon the grounds of constancy of characters, has the advantage of being founded upon a fact capable of being ascertained. On the other hand, the induction which proves it may disprove it also. The same applies to those definitions of the term wherein the phænomena of hybridism play a part.
XXV.
The balance of inconveniences is, in the mind of the present writer in favour of the idea of descent determining the meaning of the word species—for human natural history at least.
XXVI.
Hence—a species is a class of individuals, each of which is hypothetically considered to be the descendant of the same protoplast, or of the same pair of protoplasts.
XXVII.