252 THE SOCIAL LIFE OF ANIMALS 



with the more fundamental distinctions of sex. Fre- 

 quently, as in man, these differences have developed 

 into fairly distinct behavior patterns for the two 

 sexes, until each sex is practically a distinct caste, 

 almost in the sense used in discussing castes among 

 the social insects. 



Sex is usually determined by differences in he- 

 redity which are associated with the combination of 

 chromosomes (37) and of the bearers of heredity 

 (genes) that are found in the sperm and egg whose 

 union gives rise to the new individual. Such deter- 

 minations occur at the time of fertilization and sex 

 is normally unaltered thereafter. 



Exceptions occur which demonstrate that for cer- 

 tain animals this normal means of sex determination 

 can be overruled by environmental differences. Many 

 of these cases are interesting and significant but their 

 full consideration here would draw us off the main 

 thread of our present discussion. We shall follow 

 only those instances in which changes in sex are 

 associated with the near-by presence of other indi- 

 viduals, considering here two widely differing cases 

 which have been carefully investigated in recent 

 years. 



Professor Coe (35) of Yale has spent much of his 

 time studying the sex ratios and sexual changes in 

 oysters, clams, marine snails and other related forms. 



