BY SAYING that a child has inherited this or that characteristic
from one of his parents, one postulates a process that provides
for continuity between one generation and the next. Indeed, con-
tinuity is the essence of the entire concept of heredity. The Greeks
already vaguely understood that sexual union was the key to the
solution of the problem of heredity, but how the "genetic mate-
rial" (as it was later called) was transmitted from one generation
to the next was entirely a matter of speculation (see Chapter 14).
Some of the proposed theories were highly improbable because
the inheritance of physical and behavioral characteristics was far
too precise and detailed to be explained in terms of "heat" or of
"pneuma" or of other generalized physical forces, as proposed by
most early philosophers. The school of Hippocrates seems to have
come much closer to the truth when it explained inheritance as
due to the transmission of seed stuff. A qualitative theory of in-
heritance was proposed by Lucretius, according to whom the
qualities of hair, voice, face, and other parts of the body are de-
termined by the mixture of atoms contained in the seed inherited
from the ancestors. All observations on inheritance suggested that
something qualitative-corpuscular was transmitted but, whatever
it was, it was far too small to be seen by the naked eye. An entirely
new branch of biology, cytology, first had to develop before the
challenge of the nature of the genetic material could be met. The
development of this new discipline was not possible until the mi-
croscope had been invented and applied to the study of cells. ¹

That eggs are necessary for the development of a new indi-
vidual had long been evident, and that the male semen is also
important was likewise already a widespread belief among the
Greeks, and more or less conceded even by most so-called ovists
in the seventeenth and eighteenth centuries. However, definite
proof was not available until the 1760s. The similarity, if not iden-
tity, of the hybrids produced in reciprocal crosses (as in the work
of Kölreuter) led to the inevitable conclusion that the genetic con-

-652-