ire.
If several trifling characters are always found in combination, though
no apparent bond of connexion can be discovered between them, especial
value is set on them. As in most groups of animals, important 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 important vital organs, are found to offer characters of quite
subordinate value. Thus, as Fritz Muller has lately remarked, in the
same group of crustaceans, Cypridina is furnished with a heart, while in
two closely allied genera, namely Cypris and Cytherea, there is no such
organ; one species of Cypridina has well-developed branchiae, while
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 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. 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 crus