This is shown most beautifully and clearly in the extraordinarily interesting phenomena called dimorphism and polymorphism. These seem to me to show how very different final results may grow from identical rudiments, if these, in early stages of development, be subjected to different external influences. Finally, I have a little to say about the sexual dimorphism that occurs so generally in the animal kingdom. Nearly all kinds of animals appear as male or as females. These differ from each other not only in that they produce eggs or spermatozoa, but frequently in a number of more or less striking characters affecting different parts of the body, and known as secondary sexual characters. In fact, the difference between the sexes may be so great that a systematic naturalist, unacquainted with the mode of development of the creatures, might place them in different species, genera, or even families, on account of the striking differences in external characters. As an instance, take _Bonellia_, a gephyrean, the strange case of which has been remarked upon by Hensen and by Weismann. The male is about a hundred times smaller than the female, in the respiratory chamber of which it lives as a kind of parasite, and appears, so far as outward shape goes, more like a turbellarian than a gephyrean. None the less, male and female are alike not only while they are in the egg, but as larvæ, and it is only towards the period of sexual maturity that the great difference between them begins to appear. So also is it with the dwarf males of the cirripedes. Males and females, whether they be more or less unlike, arise from the same germinal material. The germinal material itself is sexless; that is to say, there is not a male and a female germinal material. The phenomena of inheritance in the sexual generation of hybrids show this clearly. Characters appropriate both to males and to females are transmitted either by eggs or by spermatozoa. In parthenogenetic animals both male and female individuals appear at definite times from eggs produced without sexual commerce. Whether the male or the female forms be produced depends, not upon any difference in the germinal material, but on the external influences, just as external influences determine whether the bud on a twig shall give rise to a vegetative or to a flowering shoot, to a thorn or to a stem. The influence of food, of temperature, or probably of other agencies, determines in which direction the germinal material shall grow. The experiments of a distinguished French investigator, M. Maupas, on the determination of sex in _Hydatina senta_, a rotifer, have given striking results.