We are now in a position to gain a clue to the difficult problem of the origin of the sexes. It would be easy as well as instructive to accumulate examples.[14] I am tempted to linger over the life-histories of these early organisms that are so full of suggestion; but the case I have selected--the _volvox_--really answers the question. Sex here is dependent on, and would seem to have arisen through, differences in environmental conditions. We find the well-nourished, larger, and usually more quiescent cell is the female, the hungrier and more mobile cell the male; the one concerned with storing energy, the other with consuming it, the one building up, the other breaking down; or expressed in biological formula, the female cell is predominantly anabolic, that of the male predominantly katabolic. Thus we find that the male, through a want of nutrition, was carried developmentally away from the well-fed female cell, which it was bound to seek and unite with to continue life. This relation between the food supply and the sexes is found persisting in higher forms, and, in this connection, the well-known experiments of Young on tadpoles and of Siebald on wasps may be cited. By increasing the nutrition of tadpoles the percentage of females was raised from the normal of about fifty per cent. to ninety, while similarly among wasps the number of females was found to depend on warmth and food supply, and to decrease as these diminished. Mention also may be made of the plant-lice, or aphides, which infest our rose-bushes and other plants, which, during the summer months, when conditions are favourable, produce generation after generation of females, but on the advent of autumn, with its cold and scarcity of food, males appear and sexual reproduction takes place. Similarly brine-shrimps when living under favourable conditions produce females, but when the environment is less favourable males as well are found. Another significant fact is the simple and well-known one that within the first eight days of larval life the additions of food will determine the striking and functional differences between the workers and queen-bee.[15] Among the higher animals the difficulties of proving the influence of environment upon sex are, of course, much greater. There are, however, many facts which point to a persistence of this fundamental differentiation. Among these it is sufficient to mention the experiments of stock-breeders, which show that good conditions tend to produce females; and the testimony of furriers that rich regions yield more furs from females, and poor regions more from males. Even when we reach the human species facts are not wanting to suggest a similar condition. It is usual in times of war and famine for more boys to be born; also more boys are born in the country than in cities, possibly because the city diet is richer, especially in meat. Similarly among poor families the percentage of boys is higher than in well-to-do families.