We can see why characters derived from the embryo should be of equal importance with those derived from the adult, for a natural classification of course includes all ages.

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.