J. W. H. Harrison 17 



Hybridity or heterozygosis, as opposed to the enervating etFects of 

 inbreeding, has a building up or stimulating influence on the hybrid 

 organism physiologically. Should progressive sterility not be displayed, 

 and it was not in the special case we are discussing, this heterozygosis 

 of the pomonaria-isahellae hybrid should so affect the sex genes as 

 rather to increase their values in comparison with the stationary factors 

 of zonaria than diminish them. 



This increased vigour, due to its heterozygous nature, had other 

 expression in the insect than in size ; it was betrayed in the greater 

 manifestation of the sexual instincts which caused all the hybrids of 

 this origin to pair instantaneously with any female with which they 

 were enclosed. The copulation with the zonaria female lasted for the 

 prolonged period of 36 hours. 



The chromosome complement of the two species P. pomonaria and 

 P. isabellae is almost alike, the former species being endowed with a 

 haploid number of 51 and the latter of 52. In the gametogenesis of 

 their hybrids so homologous are the chromosomes that almost uniformly 

 a perfect reduction division takes place. Even had it not done so, that 

 fact, by- its very occurrence, would give us an extremely forceful argu- 

 ment against any interference with the anticipated sex yield of unisexual 

 male broods only. In the event of no reduction, the spermatozoa of the 

 hybrid male, in lieu of being supplied with one male sex gene or chromo- 

 some, would carry two, and as a necessary consequence would be doubly 

 endowed with factors active in upfjetting the power of the female sex 

 determiner in the female-producing ova of N. zonaria. 



Again, this unlooked-for female was quite abnormal in size ; it 

 barely attained one half that of the pure species. To make this the 

 more emphatic the insect was weighed, as were also males and females 

 of the pure form and, to ensure accuracy, the weighing was undertaken 

 while the insects were as yet in the pupal state. The following were 

 the respective weights : 



Weight of abnormal hybrid female . . . '14 grm. 



Weight of hybrid males '23 grm. 



Mean weight of the females of P. pomonaria, P. 



isabellae, and N. zonaria '27 grm. 



From this table it is apparent that the female in question is practically 

 one half of the weight one would have anticipated. This argues or 

 rather indicates that something has occurred to prevent the develop- 

 ment of the insect to its full size. What caused this phenomenon was, 

 Journ. of Gen. ix 2 



