rior of the blood-vessels, to be exactly the same thing as air--it
was "a subtilized and condensed air."[11] And this we now know to be
oxygen.

The treatise "On the Generation of Animals" is an extraordinary
production. "No ancient and few modern works equal it in
comprehensiveness of detail and profound speculative insight. We here
find some of the obscurest problems of biology treated with a mastery
which, when we consider the condition of science at that day, is truly
astounding. That there are many errors, many deficiencies, and not a
little carelessness in the admission of facts, may be readily imagined;
nevertheless at times the work is frequently on a level with, and
occasionally even rises above, the speculations of many advanced
embryologists."[12]

It commences with the statement that the present work is a sequel to
that "On the Parts of Animals;" and first the masculine and feminine
_principles_ are defined. The masculine principle is the origin of all
motion and generation; the feminine principle is the origin of the
material generated. Aristotle's philosophy of nature was teleological,
and the imperfect character of his anatomical knowledge often gives him
occasion to explain particular phenomena by final causes. Thus animals
producing soft-shelled eggs (_e.g._ cartilaginous fish and vipers) are
said to do so because they have so little warmth that the external
surface of the egg cannot be dried.

Among insects, some (_e.g._ grasshopper, cricket, ant, etc.) produce
young in the ordinary way, by the union of the sexes; in other cases
(_e.g._ flies and fleas) this union of the sexes results in the
production of a _skolex_; while others have no parents, nor do they have
congress--such are the ephemera, tipula, and the like. Aristotle
discusses and rejects the theory that the male reproductive element is
derived from every part of the body. He concludes that "instead of
saying that it comes _from_ all parts of the body, we should say that it
goes _to_ them. It is not the nutrient fluid, but that which is _left
over_, which is secreted. Hence the larger animals have fewer young than
the smaller, for by them the consumption of nutrient material will be
larger and the secretion less. Another point to be noticed is, that the
nutrient fluid is universally distributed through the body, but each
secretion has its separate organ.... It is thus intelligible why
children resemble their parents, since that which makes