According to Pflüger, there are normally found amongst frogs in a state of nature 36.3 per cent. males, and 63.7 per cent. females. With thinner or thicker semen, the average number can be altered. With thin semen Pflüger obtained 27.3 males and 72.7 females per cent. With concentrated semen he obtained 39.4 males, and 60.6 females. Pflüger carried out some other experiments, and came to the conclusion that the concentration of the semen or the extract from the testicles exercised either a very small influence or no influence at all upon the sex. When he took an average result from all his experiments, he found that out of 806 frogs which he raised, 288 were males. Whilst the normal proportion of the males developed freely under natural conditions was 36.3 per cent., that reached by experiment was 35.7 per cent. It should be here remarked that among the tadpoles many are found whose sex is not yet determined. They are in a hermaphrodite condition, out of which they develop into either males or females. * * * * * Robin, the well-known French histologist, made the question of the origin of sex the subject of an extensive inquiry. His investigations start from the following point:--In warm climates the whole quantity of blood in the inhabitants is less than in temperate zones. The process of respiration in the inhabitants of warm climates is also not so free as in the case of those who inhabit temperate or cold zones. From this it would seem that some process connected with nutrition, and with the passage of nourishment into the blood, is the cause of the number of male births being greater in the cold zones than it is in the temperate zones, or in the inhabited regions lying nearer to the equator. It would follow from these considerations, that if the women were subjected to such a régime as would materially affect their respiration and the quality of their blood, more boys than girls, or the contrary, might be bred. If so, breathing an atmosphere containing more oxygen, with a corresponding diet, would be the right receipt for producing in the woman such a basis that in the course of development the male generative organs (which Robin considers the anatomically more perfectly developed) might be evolved instead of the female. According to Robin, the male sexual apparatus in comparison with that of the female is provided with the more perfectly developed character. Robin further insists that strong men will beget more male individuals. Further, that a woman who indulges in sexual intercourse somewhat seldom has female children; and that voluptuous women, who are fruitful, generally bear boys. A number of experiments with domesticated mammalia are adduced in support of this view. * * * * * Hegar teaches that in the case of a merely rudimentary development of the germinal gland either sex is developed.