We must come down to the last century, to Linnaeus, before we find the history taken up where Aristotle had left it, and some of his suggestions carried out with new vigor and vitality.  Aristotle had distinguished only between genera and species; Linnaeus took hold of this idea, and gave special names to other groups, of different weight and value.  Besides species and genera, he gives us orders and classes,—­considering classes the most comprehensive, then orders, then genera, then species.  He did not, however, represent these groups as distinguished by their nature, but only by their range; they were still to him, as genera and species had been to Aristotle, only larger or smaller groups, not founded upon and limited by different categories of structure.  He divided the animal kingdom into six classes, which I give here, as we shall have occasion to compare them with other classifications:—­Mammalia, Birds, Reptiles, 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.  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