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. 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.
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