254 THE THEORY OF THE GENE 



larva attaches itself to the proboscis of a female it remains 

 extremely small and develops testes, but if it settles down 

 by itself it becomes a large female individual. The evi- 

 dence does not positively rule out the possibility that 

 there are two kinds of individuals that behave in one or 

 the other way, but Baltzer's interpretation seems very 

 probable. 



If the correct interpretation for the barnacles and for 

 Bonellia is that suggested above, it means that sex is 

 determined in these forms by environmental conditions, 

 which means, in terms of genes, that all the individuals 

 are alike. 2 



Changes of Sex Associated with Age. 



Biologists are familiar with several cases both in ani- 

 mals and in plants where an individual may first function 

 as a male and later as a female, or vice versa. But the 

 special cases in which sex reversal takes place are those 

 whose sex is known to be determined in the first place 

 by their chromosomal make-up, yet which are said, in 

 rare cases, to turn into the opposite sex without changing 

 their chromosome complex. 



The hagfish, Myxine, according to Nansen and Cun- 

 ningham, is male when young, and later becomes female ; 

 but the subsequent observations of the Schreiners indi- 

 cate that while the young Myxine is hermaphroditic — the 

 anterior end of the gonad being a testis, the posterior 

 an ovary — it is not so functionally. Later each individual 

 becomes definitively male or female. 



Breeders of the aquarium fish, Xiphophorus helleri, 

 have reported at various times that females change into 

 males, but, unfortunately, as yet there is no account of 



2 According to Gould, if a young individual of Crepidula plana settles 

 down near a female it becomes at first a male and remains so permanently; 

 but if it settles down away from large individuals it fails to develop testes 

 and passes later into a female. 



