But in giving full meed of praise to Aristotle for the
promulgation of this doctrine of the sphericity of the earth, it
must unfortunately be added that the conservative philosopher
paused without taking one other important step. He could not
accept, but, on the contrary, he expressly repudiated, the
doctrine of the earth's motion. We have seen that this idea also
was a part of the Pythagorean doctrine, and we shall have
occasion to dwell more at length on this point in a succeeding
chapter. It has even been contended by some critics that it was
the adverse conviction of the Peripatetic philosopher which, more
than any other single influence, tended to retard the progress of
the true doctrine regarding the mechanism of the heavens.
Aristotle accepted the sphericity of the earth, and that doctrine
became a commonplace of scientific knowledge, and so continued
throughout classical antiquity. But Aristotle rejected the
doctrine of the earth's motion, and that doctrine, though
promulgated actively by a few contemporaries and immediate
successors of the Stagirite, was then doomed to sink out of view
for more than a thousand years. If it be a correct assumption
that the influence of Aristotle was, in a large measure,
responsible for this result, then we shall perhaps not be far
astray in assuming that the great founder of the Peripatetic
school was, on the whole, more instrumental in retarding the
progress of astronomical science that any other one man that ever
lived.

The field of science in which Aristotle was pre-eminently a
pathfinder is zoology. His writings on natural history have
largely been preserved, and they constitute by far the most
important contribution to the subject that has come down to us
from antiquity. They show us that Aristotle had gained possession
of the widest range of facts regarding the animal kingdom, and,
what is far more important, had attempted to classify these
facts. In so doing he became the founder of systematic zoology.
Aristotle's classification of the animal kingdom was known and
studied throughout the Middle Ages, and, in fact, remained in
vogue until superseded by that of Cuvier in the nineteenth
century. It is not to be supposed that all the terms of
Aristotle's classification originated with him. Some of the
divisions are too patent to have escaped the observation of his
predecessors. Thus, for example, the distinction between birds
and fishes as separate classes of animals is so obvious that it
must appeal to a child or to a savage. But the efforts of
Aristotle extended, as we shall see, to less patent
generalizations. At the very outset, his grand division of the
animal kingdom into blood-bearing and bloodless animals implies a
very broad and philosophical conception of the entire animal
kingdom. The modern physiologist does not accept the
classification, inasmuch as it is now known that colorless fluids
perform the functions of blood for all the lower organisms. But
the fact remains that Aristotle's grand divisions correspond to
the grand divisions of the Lamarckian system--vertebrates and
invertebrates-- which every one now accepts. Aristotle, as we
have said, based his classification upon observation of the
blood; Lamarck was guided by a study of the skeleton. The fact
that such diverse points of view could direct the observer
towards the same result gives, inferentially, a suggestive lesson
in what the modern physiologist calls the homologies of parts of
the organism.

Aristotle divides his so-called blood-bearing animals into five
classes: (1) Four-footed animals that bring forth their young
alive; (2) birds; (3) egg-laying four- footed animals (including
what modern naturalists call reptiles and amphibians); (4) whales
and their allies; (5) fishes. This classification, as will be
observed, is not so very far afield from the modern divisions
into mammals, birds, reptiles, amphibians, and fishes. That
Aristotle should have recognized the fundamental distinction
between fishes and the fish- like whales, dolphins, and porpoises
proves the far from superficial character of his studies.
Aristotle knew that these animals breathe by means of lungs and
that they produce living young. He recognized, therefore, their
affinity with his first class of animals, even if he did not,
like the modern naturalist, consider these affinities close
enough to justify bringing the two types together into a single
class.

The bloodless animals were also divided by Aristotle into five
classes--namely: (1) Cephalopoda (the octopus, cuttle-fish,
etc.); (2) weak-shelled animals (crabs, etc.); (3) insects and
their allies (including various forms, such as spiders and
centipedes, which the modern classifier prefers to place by
themselves); (4) hard-shelled animals (clams, oysters, snails,
etc.); (5) a conglomerate group of marine forms, including
star-fish, sea-urchins, and various anomalous forms that were
regarded as linking the animal to the vegetable worlds. This
classification of the lower forms of animal life continued in
vogue until Cuvier substituted for it his famous grouping into
articulates, mollusks, and radiates; which grouping in turn was
in part superseded later in the nineteenth century.

What Aristotle did for the animal kingdom his pupil,
Theophrastus, did in some measure for the vegetable kingdom.
Theophrastus, however, was much less a classifier than his
master, and his work on botany, called The Natural History of
Development, pays comparatively slight attention to theoretical
questions. It deals largely with such practicalities as the
making of charcoal, of pitch, and of resin, and the effects of
various plants on the animal organism when taken as foods or as


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