In drawing up the several series he trusts chiefly to embryological characters, but receives aid from homologous and rudimentary organs, as well as from the successive periods at which the various forms of life are believed to have first appeared in our geological formations.

He has thus boldly made a great beginning, and shows us how classification will in the future be treated.

We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organisation.