482 HEREDITY AND SEX 



is very manifold, and it may be that sex is determined by a variety 

 of factors operative in different cases and at different stages. 



§ 5. First Theory: — That environmental influences, operating on 

 the sexually undetermined offspring (after fertilisation), may 

 at least have a share in determining the sex 



In many 3'oung organisms it is for a time impossible to 

 distinguish the sexes, and the assumption is often made that 

 there is a prolonged indeterminateness as regards sex. The 

 first theory that we need discuss is, as stated above, that en- 

 vironmental influences give the bias towards maleness or female- 

 ness. 



In support of this theory it has been customary to refer to 

 the interesting experiments on tadpoles made by Professor 

 Emile Yung, of Geneva, and although these are not so convincing 

 as some have thought, it is due to this zoologist to recognise 

 that he began experimental investigation of the subject at a 

 time when this mode of approach was little thought of. 



Let us recall some of Yung's evidence. Tadpoles are said to 

 linger for some time in a state of sex-indifference or potential 

 hermaphroditism. In normal conditions there are about 57 

 females to 43 males in the hundred. But tadpoles fed with 

 beef, fish and frog-flesh, yielded respectively 78, 81 and 92 

 females in a hundred. This was, of course, a very interesting 

 result, but it has been pointed out that Yung did not pay sufficient 

 attention to differential mortality, that he had not sufficiently 

 large numbers, and that although some tadpoles are potentially 

 hermaphrodite (with testes around the ovaries), there are others 

 which are quite distinctly male or female even in young stages. 

 But the most important criticism is the first, which leads Beard, 

 for instance, to say that Yung's experiments are only of import- 

 ance in regard to the relative viability of the two sexes. It is 

 necessary to have renewed experiments on a large scale, and to 



