_Fishes_, _Insects_, _Worms_.

That this classification should have expressed all that was known in the
last century of the most general relations among animals only shows how
difficult it is to generalize on such a subject; nor should we expect to
find it an easy task, when we remember the vast number of species (about a
quarter of a million) already noticed by naturalists. Linnæus 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 Linnæus, all of
which led to a clearer insight into the true relations among animals.
Linnæus himself, in his last edition of the "Systema Naturæ," 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