m, even in the vaguest way. For ages man must have been
more or less consciously improving his domesticated races of animals and
plants, yet it is not until the time of Aristotle that we have clear
evidence of any hypothesis to account for these phenomena of heredity. The
production of offspring by man was then held to be similar to the
production of a crop from seed. The seed came from the man, the woman
provided the soil. This remained the generally accepted view for many
centuries, and it was not until the recognition of woman as more than a
passive agent that the physical basis of heredity became established. That
recognition was effected by the microscope, for only with its advent was
actual {2} observation of the minute sexual cells made possible. After more
than a hundred years of conflict lasting until the end of the eighteenth
century, scientific men settled down to the view that each of the sexes
makes a definite material contribution to the offspring produced by their
joint efforts. Among animals the female contributes the ovum and the male
the spermatozoon; among plants the corresponding cells are the ovules and
pollen grains.

As a general rule it may be stated that the reproductive cells produced by
the female are relatively large and without the power of independent
movement. In addition to the actual living substance which is to take part
in the formation of a new individual, the ova are more or less heavily
loaded with the yolk substance that is to provide for the nutrition of the
developing embryo during the early stages of its existence. The size of the
ova varies enormously in different animals. In birds and reptiles where the
contents of the egg form the sole resources of the developing young they
are very large in comparison with the size of the animal which lays them.
In mammals, on the other hand, where the young are parasitic upon the
mother during the earlier stages of their growth, the eggs are minute and
only contain the small amount of yolk that enables them to reach the stage
at which they develop the processes for attaching themselves to the wall of
the maternal uterus. But whatever the differences in the size and
appearance of the ova produced by different {3} animals, they are all
comparable in that each is a distinct and separate sexual cell which, as a
rule, is unable to develop into a new individual of its species unless it
is fertilised by union with a sexual cell produced by the male.