In _Hydatina_, under normal conditions the eggs of certain individuals give rise always to males, of others always to females. By raising or lowering the temperature at the time when the eggs are being formed in the germaria of the young females, the experimenter is able to determine whether these eggs shall give rise to males or to females. After that early time the character of the egg cannot be altered by food, light, or temperature. In one experiment, in which five females not yet fully grown were kept in a room at the temperature of 26 to 28 degrees centigrade, Maupas found that, of 104 eggs only 3 per cent. gave rise to females, while in the case of other five young females of the same brood, but kept in a cold chamber at a temperature of 14 to 15 degrees centigrade, 95 per cent. of females were produced. In another experiment, young animals were kept for a few days in the cold, and then, until death, in a higher temperature. Of the eggs produced while in the cold, 75 per cent. produced females, of those deposited in the warmth, 81 per cent. became males. With these results may be compared what happens with many plants. Melons and cucumbers, which produce on the same stem both male and female flowers, bear only male flowers in high temperatures, only female flowers when subjected to cold and damp. In the case of many insects in which parthenogenesis occurs, the determination of sex depends upon fertilisation. Thus, among bees, unfertilised eggs give rise to drones, fertilised eggs to females. Sexual dimorphism in still another way reveals the intimate interactions existing between all the parts of an organism in every stage of development. It is well known, for instance, that among animals the early removal or destruction of the sexual organs hinders the development of the secondary sexual characters, or even may occasion the appearance of the characters of the other sex. Old hens become cock-feathered; human eunuchs have the high-pitched voice and the peculiarities of the larynx found in women. As much as sexual dimorphism, the phenomena of polymorphism show the enormous influence exerted by external forces upon correlated variation of the parts during development, and in this way upon the final structure. In the question of polymorphism it is worth while to discuss at some length the extreme polymorphism exhibited in the case of some of the colonial animals--first, because the matter has recently occasioned an important controversy between Herbert Spencer and Weismann; and, secondly, because the discussion will serve to make still more clear the difference between my views and those of Weismann upon the nature of the process of development. Among the colonial insects there arise, in addition to males and females, sexless individuals known as neuters. These in certain cases are very different from both males and females in structure and in social instincts.