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.
There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the Articulata.