The rules for classifying will no doubt become simpler when we have a definite object in view.

We possess no pedigree or armorial bearings; and we have to discover and trace the many diverging lines of descent in our natural genealogies, by characters of any kind which have long been inherited.

Rudimentary organs will speak infallibly with respect to the nature of long-lost structures.

Species and groups of species which are called aberrant, and which may fancifully be called living fossils, will aid us in forming a picture of the ancient forms of life.