COURTSHIP AND SEX 239 



The agreement between Riddle's experimental results 

 and the interpretations offered by Geddes and Thomson in 

 " The Evolution of Sex " (1889) is remarkable. Dr. Riddle 

 writes : "A general classification of male and female adult 

 animals on the basis of a higher metabolism for the one and 

 a lower for the others, was indeed made by Geddes and 

 Thomson many years ago. It now seems beyond question 

 that this conclusion of these authors is a correct and im- 

 portant one." It should be added that the argument of 

 " The Evolution of Sex " (1889) included some evidence 

 that the determination of sex in the individual was dependent 

 on influences affecting the metabolic level. Conditions 

 favouring an increase of the ratio of anabolic to katabolic 



/A\ 

 process (^j were held to favour the production of female 



offspring, or the development of a germ into a female 

 organism. Conversely, when the denominator of the 



A 



fraction ^^ is nearer the numerator in amount, maleness will 



be favoured. 



The general thesis, then, is that male pigeons arise from 

 eggs (yolks) of lesser storage metabolism (small size and 

 higher metabolism) and female pigeons from eggs (yolks) of 

 greater storage metabolism (large size and lower metabolism). 

 An interesting corroboration has been furnished by Riddle 

 (191 8). He obtained two cases of identical twins which 

 were both females, and which he argues were developed 

 from eggs of high storage metabolism. The proof of this 

 must be indirect, since the eggs developed ; but it was known 

 that the eggs were very large compared with all the others 

 (116 and 134) produced by the particular birds in question. 

 If male identical twins were to be found developing from a 

 very small egg it would be an interesting confirmation. It is 

 suggested that the borders of the blastoderm or embryonic 

 disc will be abnormally raised in extraordinarily large eggs 

 and abnormally lowered in extraordinarily small ones, and 

 that this might be the physical cause of the establishment of 

 two independent foci of development, yielding identical twins. 



