But it is by no means obvious, on the ordinary view, why the structure of the embryo should be more important for this purpose than that of the adult, which alone plays its full part in the economy of nature.

Yet it has been strongly urged by those great naturalists, Milne Edwards and Agassiz, that embryological characters are the most important of all; and this doctrine has very generally been admitted as true.

Nevertheless, their importance has sometimes been exaggerated, owing to the adaptive characters of larvae not having been excluded; in order to show this, Fritz Muller arranged, by the aid of such characters alone, the great class of crustaceans, and the arrangement did not prove a natural one.
But there can be no doubt that embryonic, excluding larval characters, are of the highest value for classification, not only with animals but with plants.

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.