If they find a character nearly uniform, and common to a great number of forms, and not common to others, they use it as one of high value; if common to some lesser number, they use it as of subordinate value.

This principle has been broadly confessed by some naturalists to be the true one; and by none more clearly than by that excellent botanist, Aug.

St.Hilaire.
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.