262 



Studies ill the Hybrid Bistoninae. Ill 



first year; however, mate L. hirtaria and P. pomonaria and the con- 

 ditions obtaining in the two parent forms are unchanged, the same 

 percentage exactly lying over. 



With this explanation, we can now proceed to plot our graph showing 

 the relationship between the phylogenetical divergence of the female 

 parent from L. hirtaria and the increased vigour of the hybrid organism 

 as expressed in its size. 



Y 



3 



-u 



C/3 



C 

 CO 



a 



If 



•- E 



(fi >M 

 cd o 



V o 

 .E ^ 

 <]> 



bo 



cd 



(5! 



Hirtaria xzpnaria 



Hirtaria x lapponaria 



Hirtaria xisabellae 



X 



•X Hirtaria xjrraecana* 



Hirtaria x pomonaria 



O Phylogenetic order 



Isabellae is on a side chain 



* This brood was not reared with the elaborate precautions taken in the other cases 

 to avoid phenotypic variation as, for obvious reasons, it is impossible to get new material 

 from Carniola. 



The correlation between the two conditions is so exact as to give as 

 the curve of relationship a straight line. 



In other words, the stimulus given to the organism by hybridity is 

 inversely proportional to the physiological affinity of the participating 

 species or, otherwise, it is directly proportional to the physiological 

 divergence, and the same holds true of all the other manifestations of 

 inciting force. Had mere " cross fertilisation " (to adapt the word to 

 include the fusing of gametes the affinities of which are more or less 

 remote) in itself been the sole inducing cause, it is exceedingly difficult 



