Thus the main divisions of flowering plants are founded on differences in the embryo--on the number and position of the cotyledons, and on the mode of development of the plumule and radicle.

We shall immediately see why these characters possess so high a value in classification, namely, from the natural system being genealogical in its arrangement.

Our classifications are often plainly influenced by chains of affinities.

Nothing can be easier than to define a number of characters common to all birds; but with crustaceans, any such definition has hitherto been found impossible.