the fact, and this chemists and physiologists are not at present
able to give us. Researches must be carried farther on the effect of
temperature, light, and water on variation, before we may hope to
reach a positive conclusion. We can only assume that the chemical
constitution of the organism at a given moment conditions the sex of
the offspring, and is itself conditioned by various factors--light,
heat, water, electricity, etc.--and that food is one of these
variables.[15] It is sufficient for our present purpose that sex is
a constitutional matter, indirectly dependent upon food conditions;
that the female is the result of a surplus of nutrition; and that the
relation reported among the lower forms persists in the human species.

In close connection with the foregoing we have the fact, reported
by Maupas,[16] that certain Infusorians are capable of reproducing
asexually for a number of generations, but that, unless the
individuals are sexually fertilized by crossing with unrelated forms
of the same species, they finally exhibit all the signs of senile
degeneration, ending in death.[17] After sexual conjugation there
was an access of vitality, and the asexual reproduction proceeded as
before. "The evident result of these long and fatiguing experiments
is that among the ciliates the life of the species is decomposed into
evolutional cycles, each one having for its point of departure an
individual regenerated and rejuvenated by sexual copulation."[18]

The results obtained by Maupas receive striking confirmation in the
universal experience of stock-breeders, that, in order to keep a breed
in health, it is necessary to cross it occasionally with a distinct
but allied variety. It appears, then, that a mixture of blood has a
favorable effect on the metabolism of the organism, comparable to that
of abundant nutrition, and that innutrition and in-and-in breeding are