Julian S. Huxley 273 



sex, but the actual organization of the other. Such individuals when 

 mated to normal individuals in normal conditions will always give a 

 preponderance of the sex other than that to which they themselves 

 belong. Further, if such an external factor modifying sex-ratio becomes 

 operative for a period of several generations, but is subsequently elimi- 

 nated, we shall get a decreasing preponderance of the one sex during 

 this period, then a preponderance of the opposite sex in the first 

 generation after the close of the period, and finally the normal sex-ratio 

 in succeeding generations. Abnormal sex-ratios may thus be produced 

 in a great variety of ways : 



(1) by differential fertilization; 



(2) by differential mortality of gametes ; 



(3) by differential mortality of zygotes ; 



(4) by the conversion of an X into a Y chromosome or vice versd 

 (extremely improbable) ; 



(5) by the overriding of the chromosome-constitution by means 

 of external influences, so that zygotes result with the facies of one sex 

 but the chromosome-constitution of the other. 



It should be noticed that wherever there is a swing-back of sex- 

 ratio, we must assume the last of these alternatives. Morgan ('19, pp. 197- 

 198) has already envisaged this. His comment is as follows: "It is 

 perhaps needless to point out that if, in a species in which sex is deter- 

 mined by a chromosome-mechanism, it were possible to change the sex 

 by other agencies in spite of the chromosome arrangement, the latter 

 relation would be entirely thrown out of gear and males would transmit 

 sex-linked characters and sex itself like females, and females like males. 

 As no such cases have been found, it is futile to discuss such a possi- 

 bility." The case of Girardinus is, however, one where this is the 

 most probable explanation ; and it therefore becomes important to test 

 other cases of abnormal sex-ratio for this possibility. In addition, the 

 phenomena in Lymantria also demand this explanation. 



