IIALDANL S RULli 



iiiality, or failure in some vital function, in wide hybrids. Much 

 more commonly these are to be traced to disharmony at the 

 third and most fundamental level, that of the genes within the 

 nucleus. The father's chromosomes and the mother's cannot work 

 together properly in the hybrid. 



HaUaiies Rule 



Disharmony, or unbalance of the genes, arising from hybridiza- 

 tion affects every nucleus of the soma, from that of the fertilized 

 egg onwards. It may therefore express itself in any stage of develop- 



Homogametic Sex X Heterogametic Sex 



AAXX X aaxy 



AaXx 



AaXy 



F, of HOMOGAMETIC sex 

 with balance intermediate 

 between that of the two 

 parents 



aaxx X AAXY 



F, of HETEROGAMETIC sex 

 with new balances different 

 in the reciprocal crosses 



FEMALE . . XX and MALE . 

 MALE ... XX and FEMALE . 



XY in Mammals and Flies 

 XY in Moths and Birds 



Fig. 54. — Diagram to show the relation of balance and fertility in reciprocal crosses 

 between species in animals. Chromosomes of opposite species shown in large and 

 small type. 



nicnt, by untimely death or by loss of vigour; or in reproduction, 

 by sterility of greater or less degree. The genie balance should be 

 least disturbed where the contributions of father and mother are 

 each complete and balanced, as in the case of a first cross between 

 two races or species. And the greater the departure from this balanced 



129 



