Seite - 456 - in The Origin of Species

456 ORIGIN OF SPECIES be the true one ; and by none more clearly than by that ex- cellent botanist, Aug. St. Hilaire. If several trifling char- acters are always found in combination, though no apparent bond of connection can be discovered between them, especial value is set on them. As in most groups of animals, impor- tant organs, such as those for propelling the blood, or for aerating it, or those for propagating the race, are found nearly uniform, they are considered as highly serviceable in classification ; but in some groups all these, the most impor- tant vital organs, are found to offer characters of quite sub- ordinate values. Thus, as Fritz Miiller has lately remarked, in the same group of crustaceans, Cypridina is furnished with a heart, whilst in too closely allied genera, namely Cypris and Cytherea, there is no such organ ; one species of Cypri- dina has well-developed branchiae, whilst another species is destitute of them. 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 exag- gerated, owing to the adaptive characters of larvae not hav- ing been excluded; in order to show this, Fritz MuUer 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 di- visions 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.