all the parts of
the body, resembles that which is left over as secretion: thus the
hand, or the face, or the whole animal pre-exists in the sperm, though
in an undifferentiated state ({adioristos}); and what each of these is
in actuality ({energeia}), such is the sperm in potentiality
({dynamei})."

In later times the two great rival theories put forward to account for
the development of the embryo have been--

(_a_) The theory of Evolution, which makes the embryo pre-existent in
the germ, and only rendered visible by the unfolding and
expansion of its organs.

(_b_) The theory of Epigenesis, which makes the embryo arise, by a
series of successive differentiations, from a simple
homogeneous mass into a complex heterogeneous organism.

The above quotation will show how closely Aristotle held to the theory
of Epigenesis; and in another place he says, "Not at once is the animal
a man or a horse, for the end is last attained; and the specific form is
the end of each development."

Spontaneous generation is nowadays rejected by science; but Aristotle
went so far as to believe that insects, molluscs, and even eels, were
spontaneously generated. It is, however, noteworthy, in view of modern
investigations, that he looked upon _putrefying_ matter as the source of
such development.

A chapter of this work is devoted to the consideration of the hereditary
transmission of peculiarities from parent to offspring.

The fifth and last book contains inquiries into the cause of variation
in the colour of the eyes and hair, the abundance of hair, the sleep of
the embryo, sight and hearing, voice and the teeth.

Widely different opinions have been held from time to time of the value
of Aristotle's biological labours. This philosopher's reputation has,
perhaps, suffered most from those who have praised him most. The praise
has often been of such an exaggerated character as to have become
unmeaning, and to have carried with it the impression of insincerity on
the part of the writer. Such are the laudations of Cuvier. To say as he
does, "Alone, in fact, without predecessors, without having borrowed
anything from the centuries which had gone before, since they had
produced nothing enduring, the disciple of Plato discovered and
demonstrated more truths and executed more scientific labours in a life
of sixty-two years than twenty centuries after him were able to do," is
of course to talk nonsens