larities 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 Regne 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
cast. He tells us this in such admirable language that I must, to do
justice to his thought, give it in his own words:--

"Si l'on considere le regne animal d'apres les principes que nous
venons de poser en se debarrassant des prejuges etablis sur les
divisions anciennement admises, en n'ayant egard qu'a
l'organisation et a la nature des animaux, et non pas a leur
grandeur, a leur utilite, au plus ou moins de connaissance que
nous en avons, ni a toutes les autres circonstances accessoires,
on trouvera qu'il existe quatre formes principales, quatre plans
generaux, si l'on peut s'exprimer ainsi, d'apres lesquels tous les
animaux semblent avoir ete modeles, et dont les divisions
ulterieures, de quelque titre que les naturalistes les aient
decorees, ne sont que des modifications assez legeres, fondees sur
le developpement ou l'addition de quelques parties, qui ne
changent rien a l'essence du plan."

[2] Sur un nouveau rapprochement a etablir entre les Classes qui
composent le Regne Animal. _Ann. Mus._, Vol. XIX.

The value of this principle was soon