March 8, 1907] 



SCIENCE 



383 



females in the human race, and has reached 

 the conclusion that the nutrition of the 

 parents is an important factor in sex de- 

 termination. His conclusion may be seri- 

 ously questioned, because other statistics 

 have given contradictory results (those of 

 Punnett, for example), and because it is 

 apparent that so many other external fac- 

 tors than food may be involved that the 

 slight difference on vs^hich Diising based his 

 conclusion may be due to other conditions 

 than nutrition. If, in reality, nutrition 

 were a factor in sex determination in man 

 we should expect to find, I think, a far 

 greater disproportion of males to females 

 in the offspring of the rich and of the poor 

 than Diising 's statistics show. If further 

 evidence is needed it is furnished by the 

 recent experiments of Cuenot with rats, 

 and of Schultze with mice. Even extreme 

 conditions of starvation and of feeding 

 produced no Effects upon the birth rate of 

 males to females. 



If there were time we might pass in re- 

 view the other external factors that have 

 been supposed to account for sex deter- 

 mination. It must suffice to state that in 

 no instance has a good case been made out 

 for any one of them. 



Our opinion in regard to the possible 

 effects of external factors in sex deter- 

 mination has been influenced by our 

 knowledge of the changes that take place 

 in the life cycle of certain animals- 

 changes that appear at first sight to be 

 associated with sex, but in reality may 

 relate to another phenomenon. In two, 

 possibly in three, groups of animals a 

 change from the parthenogenetic to the 

 sexual mode of reproduction appears to be 

 associated with changes in the environ- 

 ment. I may mention first the aphids, 

 since I have studied the problem in this 

 group. As is well known, parthenogenesis 

 is the rule during the summer, but in the 

 autumn the sexual forms appear. Cold 



does not bring about the result and it is 

 almost certain that the change is incited by 

 food conditions. The important fact to 

 note is that, although an external factor 

 causes the appearance of the males, it does 

 so by introducing a new method of repro- 

 duction in which both males and sexual 

 females appear. 



A somewhat similar result has been 

 found in Daphnia, where, also, according 

 to the recent results of Issakowitsch, lack 

 of food causes parthenogenetic reproduc- 

 tion to cease, and both males and sexual 

 eggs to appear. 



For the rotifer, Hydatina senta, Maupas 

 has claimed that temperature regulates 

 sex, while Nussbaum has tried to show that 

 food is responsible for the result. Quite 

 recently Punnett has discovered that the de- 

 tei-mining condition is not external at all, 

 but that there are male strains and female 

 strains, that give rise to their particular 

 sex independently of the environment. 



In the light of the evidence that we have 

 at present it seems probable then that, in 

 the higher animals at least, sex is deter- 

 mined by internal, not by external, factors. 

 What the nature of the internal mechanism 

 may be we do not know, but it is a curious 

 and significant fact that in modern at- 

 tempts to account for the nature of the 

 change that takes place, the biologist finds 

 himself trying once more to steer his course 

 between the inevitable alternatives of pre- 

 formation and epigenesis. The history of 

 our science has shown, in fact, that pre- 

 formation and epigenesis are two poles of 

 thought between which speculation contin- 

 ually and necessarily vacillates. 



One school, the preformationists, as- 

 sumes that only the male or the female 

 characters are carried by each egg or 

 sperm, hence sex is-^preformed in the sense 

 that its primordia are separated and come 

 to lie in different germ cells. 



The opposite school, that of epigenesis, 



