We can clearly see how it is that all living and extinct forms can be grouped together within a few great classes; and how the several members of each class are connected together by the most complex and radiating lines of affinities.

We shall never, probably, disentangle the inextricable web of the affinities between the members of any one class; but when we have a distinct object in view, and do not look to some unknown plan of creation, we may hope to make sure but slow progress.
Professor Haeckel in his "Generelle Morphologie" and in another works, has recently brought his great knowledge and abilities to bear on what he calls phylogeny, or the lines of descent of all organic beings.
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.
MORPHOLOGY.
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.

This resemblance is often expressed by the term "unity of type;" or by saying that the several parts and organs in the different species of the class are homologous.

The whole subject is included under the general term of Morphology.