36 



THE EVOLUTION OF SEX. 



CHAPTER IV. 



THE DETERMINATION OF SEX. 



{Experiment a?id Rationale.} 



Influence of Nutrition. — Throughout Nature the influence 

 of food is undoubtedly one of the most important environmental 

 factors. To Claude Bernard, indeed, the whole problem of evolution 

 was very much a question of variations of nutrition. " L' evolution, 

 c'est 1' ensemble constant de ces alternatives de la nutrition ; c'est 

 la nutrition considered dans sa realite, embrassee d'un coup d'oeil a 

 traverse le temps." It is fitting that we should begin our survey 

 of the factors known to influence sex with the fundamental function 

 of nutrition. 



(a) The Case of Tadpoles. — Not a few investigaters who have 

 passed from statistics and hypothesis to experiment and induction, 

 have found their materiel in tadpoles, where the sex seems to remain 

 for a comparatively long period indeterminate. If we take the 

 verdict of Yung, who has had most experience with these forms, 

 tadpoles pass through a hermaphrodite stage, in common, according 

 to other authorities, with most animals. During this phase external 

 influences, and especially food, decide their fate as regards sex, though 

 the hermaphroditism, as we shall afterwards see, sometimes per- 

 sists in adult life. It is fair, however, to notice that Pfliiger gives 

 a somewhat different account of the actual facts, distinguishing among 

 tadpoles three varieties — (a) distinct males, (6) distinct females, and 

 (f) hermaphrodites. In the last, testes, or male organs, develop 

 round primitive ovaries, and if the tadpoles are to become males the 

 inclosed female organs are absorbed. 



Adopting the view stated by Yung, we shall simply state the 

 striking results of one series of observations. When the tadpoles 

 were left to themselves, the percentage of females was rather in the 

 majority. In three lots the proportions of females to males were 

 as follows: 54-46; 61-39; ar >d 56-44. The average number of 

 females was thus about 57 in the hundred. In the first brood, by 

 feeding one set with beef, Yung raised the percentage of females 

 from 54 to 78; in the second, with fish, the percentage rose from 

 61 to 81; while in the third set, when the especially nutritious flesh 

 of frogs was supplied, the percentage rose from 56 to 92. That is 

 to say, in the last case the result of high feeding was that there were 

 92 females to 8 males. From the experience and carefulness of the 

 observer, these striking results are entitled to great weight. 



