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.

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.

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.