Similarly, the experiments of Siebold on wasps show that the percentage of females increases from spring to August, and then diminishes.  We may conclude without scruple that the production of females from fertilized ova increases with the temperature and food supply, and decreases as these diminish.[12]

Nor are there many facts more significant than the simple and well-known one that within the first eight days of larval life the addition of food will determine the striking and functional differences between worker and queen.[13]

It is certainly no mere chance, but agrees with other well-known facts, that for the generation of the female organ more favorable external circumstances must prevail, while the male organ may develop under very much more unfavorable conditions.[14]

These facts are not conclusive, but they all point in the same direction, and are probably sufficient to establish a connection between food conditions and the determination of sex.  But behind the mere fact that a different attitude toward food determines difference of sex lies the more fundamental—­indeed, the real—­explanation of 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 alike prejudicial.