COURTSHIP AND SEX 233 



§ 13. What determines Sex? 



Many answers have been given to the question : What 

 determines whether a fertilised egg-cell will develop into a 

 male or a female organism ? But the only answers that can 

 be considered seriously are the following : — 



(I.) Possible Influence of Nurture on the developing 

 Offspring. — In some organisms the sex seems to remain 

 undecided for some time, and the conditions of " nurture," 

 e.g. diet, seem to sway it to one side or to the other. Thus 

 one of the most instructive of recent biological stories con- 

 cerns a remarkable green worm called Bonellia, which is well 

 known in the Mediterranean. The female has a flask- 

 shaped body an inch or two in length, and a flexible bifid 

 proboscis much longer. The male is very simple in structure, 

 the merest pigmy in size, a hundred times smaller than his 

 mate on whom he lives as a parasite. Now the zoologist 

 Baltzer has shown that if the very young sexually indifferent 

 larvae of Bonellia, just hatched from the eggs, happen to 

 become attached to the proboscis of an adult female, they 

 develop into males ; whereas, if they fail to attach themselves 

 and sink into the sand or mud, they develop slowly into 

 females — ^with veiy few exceptions. But the story does not 

 end here. Baltzer helped some of the very young free- 

 swimming Bonellias to attach themselves to the proboscis 

 of a full-grown female ; those that he left attached for a 

 very short time developed into almost perfect females ; those 

 that he left attached for a long time became perfect males, 

 if such degenerate pigmies can be called perfect ; while 

 those which he left attached for intermediate periods showed 

 various stages of inter-sex. These fine experiments seem to 

 point to a conception of sex as something constitutional, 

 quantitative, plastic, and even reversible. If the metaphor 

 be permissible, the radical difference between the sexes is 

 one of physiological gearing. In the terminology of " The 

 Evolution of Sex " (1889) the ratio of anabolism to kata- 

 bolism is higher in the female than in the male. 



It must be remembered that there are not a few animals 



