SEX IN ANIMALS 



543 



exactly the same as that among Miss King's rats, 119.2 males to 100 

 females. 



Among wide crosses of hybrid birds the percentage of males also 

 appears to be abnormally high, but this can hardly be taken as in 

 conformity with the data of King and Minot, because in birds the mode 

 of sex-inheritance is exactly the reverse of that in mammals. Ac- 

 cordingly these data should be regarded as confirmatory of the data 

 from bison-cattle crosses, and those of Detlefsen with cavy species 

 crosses. 



As another possible source of variation in the sex-ratio, mention 

 must be made of the time of service with respect to the inception of the 

 period of heat. Various theories of sex have from time to time been 

 founded on heat relations, some maintaining that the products of con- 

 ception in early heat were more often males, others that they were more 

 often females. Pearl and Parshley have made an experimental-statisti- 

 cal study of this question, the data of which are given in Table LXXII. 

 These data were collected from farmers in the state of Maine, and 

 represent all breeds and ages of cattle. Pearl and Parshley draw 

 attention to the fact that they show a steady increase in the proportion 

 of male births with later coitus. The question as to the general signifi- 

 cance of this fact, they discuss at some length. In Table LXXIII is 

 given the statistical treatment of these data. Only in one case, that 



TABLE LXXII. THE EFFECT OF SERVICE AT DIFFERENT PERIODS OF HEAT ON THE 

 SEX-RATIO IN CATTLE (Data of Pearl and Parshley) 



TABLE LXXIII. STATISTICAL TREATMENT OF THE SEX-RATIOS OF TABLE LXXII 

 (Data of Pearl and Parshley) 



