to the recognition of one most important principle,--that such groups are
founded, not on external appearance, but on internal structure, and that
internal structure, therefore, is the thing to be studied. The group of
Quadrupeds was not the only defective one in this classification of
Linnæus; his class of Worms, also, was most heterogeneous, for he included
among them Shell-Fishes, Slugs, Star-Fishes, Sea-Urchins, and other
animals that bear no relation whatever to the class of Worms.

But whatever its defects, the classification of Linnæus was the first
attempt at grouping animals together according to certain common
structural characters. His followers and pupils engaged at once in a
scrutiny of the differences and similarities among animals, which soon led
to a great increase in the number of classes: instead of six, there were
presently nine, twelve, and more. But till Cuvier's time there was no
great principle of classification. Facts were accumulated and more or less
systematized, but they were not yet arranged according to law; the
principle was still wanting by which to generalize them and give meaning
and vitality to the whole. It was Cuvier who found the key. He himself
tells us how he first began, in his investigations upon the internal
organization of animals, to use his dissections with reference to finding
the true relations between animals, and how, ever after, his knowledge of
anatomy assisted him in his classifications, and his classifications threw
new light again on his anatomical investigations,--each science thus
helping to fertilize the other. He was not one of those superficial
observers who are in haste to announce every new fact that they chance to
find, and his first paper[2] specially devoted to classification gave to
the world the ripe fruit of years of study. This was followed by his great
work, "Le Règne Animal." He said that animals were united in their most
comprehensive groups, not on special characters, but on different _plans
of structure_,--moulds, he called them, in which all animals had been