principles, he yet stimulated a search into the closer affinities among
animals by the differences he pointed out. He divided the animal kingdom
into two groups, which he called _Enaima_ and _Anaima_, or animals with
blood and animals without blood. We must remember, however, that by the
word _blood_ he designated only the red fluid circulating in the higher
animals; whereas a fluid akin to blood exists in all animals, variously
colored in some, but colorless in a large number of others.

After Aristotle, a long period elapsed without any addition to the
information he left us. Rome and the Middle Ages gave us nothing, and even
Pliny added hardly a fact to those that Aristotle recorded. And though the
great naturalists of the sixteenth century gave a new impulse to this
study, their investigations were chiefly directed towards a minute
acquaintance with the animals they had an opportunity of observing,
mingled with commentaries upon the ancients. Systematic Zoölogy was but
little advanced by their efforts.

We must come down to the last century, to Linnæus, 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; Linnæus 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_,