Page:Popular Science Monthly Volume 16.djvu/182

 of the embryo?" The other question is, "What is the process and what are the steps by which sex was developed and then gradually differentiated in the evolution of the organic kingdom?" The one is the genesis of sex in ontogeny; the other the genesis of sex in phylogeny. It is this latter question which I wish to bring before you to-day.

The two questions, however, though distinct, are yet closely related. The ontogeny is a rapid recapitulation of the main points of the phylogeny. As in the former, sex was developed out of a primitive sexless condition of the embryo, so in the latter the sexed condition so universal now among mature organisms was evolved out of a primitive sexless condition of the organic kingdom. In the ontogeny some of the conditions which determine sex have been determined and others surmised. In some animals, as, for example, in some insects and crustaceans, the fact of fertilization or non-fertilization determines with certainty the sex, as proved by the well-known observations of Siebold and others on parthenogenesis. In others it is probably the degree of maturity of the ovule at the moment of fertilization that determines it, as shown by the experiments of Cornaz under the direction of Thury. In still others, as, for example, in butterflies, it seems to be the kind and degree of nutrition of the larvæ, as shown by the observations of Mrs. Treat. In still others it may be the prepotency of the one parent or the other, or still other causes wholly unknown. In any case, however, the subject lies fairly within the domain of science; the conditions will eventually be discovered, and, being known, will be artificially arranged so as to determine the one sex or the other with certainty.

But this is not the question which now concerns us, for we have already discussed this in a previous lecture. We wish in this lecture to show that, in the history of the organic kingdom also, sex has been gradually evolved out of a primitive sexless condition, and if possible to catch some glimpses of the main steps of the process. The most important steps are indeed very obscure; but this is only because these are among the very earliest steps of evolution.

—Now, the law under which I wish to bring the process of evolution of sex is that most universal of all the laws of evolution, viz., the law of differentiation. We have already explained to you and illustrated in many ways how, from an almost unorganized condition, in which every part is like every other part, and each part performs in an imperfect manner all the functions necessary to life—how, I say, from this primitive generalized condition, the several organs were gradually differentiated, the several functions separated and localized, and thus the complex work of the body parceled out by division of labor, until in the