XXIII.

The definition of the term species by means of the idea of descent from a single protoplast, has the advantage of being permanent and immutable; inasmuch as it is based upon a ground that no subsequent change can set aside--

"----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.