i68 THE POPULAR SCIENCE MONTHLY. 



velopment 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 be- 

 fore you to-day. 



The two questions, however, though distinct, are yet closely re- 

 lated. The ontogeny is a rapid recapitulation of the main points of 

 the phylogeny. As in the former, sex was developed out of a primi- 

 tive sexless condition of the embryo, so in the latter the sexed condi- 

 tion 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 deter- 

 mines with certainty the sex, as proved by the well-known observa- 

 tions of Siebold and others on parthenogenesis. In others it is prob- 

 ably 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 larvae, as shown by 

 the observations of Mrs. Treat.f In still others it may be the prepo- 

 tency of the one parent or the other, or still other causes wholly un- 

 known. In any case, however, the subject lies fairly within the do- 

 main 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. 



2. The General Law under which the Process falls. — 

 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 laio 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 primi- 

 tive generalized condition, the several organs were gradually differen- 

 tiated, the several functions separated and localized, and thus the com- 

 plex work of the body parceled out by division of labor, until in the 



* " Biblioth^que Universelle," September, 1863. 



f "American Naturalist," 1873 ; "Popular Science Monthly," June, 1873, 



