rter of a million) already noticed by naturalists. Linnaeus succeeded,
however, in finding a common character on which to unite most of his
classes; but the Mammalia, that group to which we ourselves belong,
remained very imperfect. Indeed, in the earlier editions of his
classification, he does not apply the name of Mammalia to this class, but
calls the higher animals _Quadrupedia_, characterizing them as the animals
with four legs and covered with fur or hair, that bring forth living young
and nurse them with milk. In thus admitting external features as class
characters, he excluded many animals which by their mode of reproduction,
as well as by their respiration and circulation, belong to this class as
much as the Quadrupeds,--as, for instance, all the Cetaceans, (Whales,
Porpoises, and the like,) which, though they have not legs, nor are their
bodies covered with hair or fur, yet bring forth living young, nurse them
with milk, are warm-blooded and air-breathing. As more was learned of
these animals, there arose serious discussion and criticism among
contemporary naturalists respecting the classification of Linnaeus, all of
which led to a clearer insight into the true relations among animals.
Linnaeus himself, in his last edition of the "Systema Naturae," shows us
what important progress he had made since he first announced his views;
for he there substitutes for the name of _Quadrupedia_ that of _Mammalia_,
including among them the Whales, which he characterizes as air-breathing,
warm-blooded, and bringing forth living young which they nurse with milk.
Thus the very deficiencies of his classification stimulated naturalists to
new criticism and investigation into the true limits of classes, and led
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
Linnaeus; 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 Linnaeus 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 simi