264 Studies m the Hybrid Bistoninae. Ill 



the greater the number of diverse determiners (so long as they are able 

 to work without upsetting the stability of the cell thereby causing it to 

 perish) the greater the stimulating effect. 



Again, the stimulus may result mainly from the presence in any 

 given cell of a greater number of units than it was designed to receive, 

 causing a development of increased capacity to allow for these extra 

 units, and thus increased and more rapid growth. 



In connection with this, it is well to note that, in reality, the size 

 of the cells in the only hybrid I have critically examined was actually 

 greater than that of either hirtaria or zonaria, the two parents. 



Lastly, it must be remarked that the Mendelian factors are not the 

 only reacting features ; there are assuredly many nuclear elements not 

 of that type, although such, very likely, predominate, and the activities 

 of this element and the resulting impulses must have some effect. 



It will be clear that, up to this stage, only heterozygotic stimulus, 

 as it appears in the F^ generation of hybrids, has been considered ; the 

 reasons for this are plain. Firstly, the F^ generations in the hybrid 

 Bistoninae are, for the most part, completely sterile, and, secondly, each 

 and every zygote generated in any particular cross possesses the same 

 intensity of heterozygosis, not only in the actual number of the opposing 

 factors, but in their quality. Every gamete thus is stimulated to exactly 

 the same extent, and, if we make due allowance for that variation which 

 would be exhibited even by a pure line, will reach approximately the 

 same size in the same time. 



But when, as in the pomonaria-isabellae hybrids, the F^ generations 

 are fertile, a new set of circumstances arises on pairing these inter se\ 

 circumstances, let it be emphasised, not those appearing in F^ insects 

 of a mono-Mendelian hybrid origin or of hybrids differing by a 

 relatively small number of opposing genes, but those developing in 

 an i^2 generation in which the interacting and conflicting genes reach 

 enormous figures. How soon, under these conditions, the number of 

 heterozygotes reaches appalling dimensions, and how quickly the homo- 

 zygotes descend to negligible figures may be gathered from the following 

 table : 



Number of 



Percentage of 

 homozygotes 



50 

 25 

 12-5 

 6-25 

 3 12 

 100 

 2" 



4» 2» 2»^{2"-l) 



