H. Onslow 291 



experiments, they have all been placed together in the column " rustica 

 and standfussi " (DD and DR) and the variation of colour must be judged 

 by reference to the curves. 



There is one more point of some interest. Both Tutt and Caradja 

 consider that rustica is phylogenetically the older form, and in this 

 Cockayne agrees, believing that a black race arising in the centre of 

 Europe has spread in all directions replacing the old white race every- 

 where except in the extreme periphery of its habitat, as in Ireland and 

 the Caucasus. In the light of present knowledge this seems to be un- 

 doubtedly correct, for if evolution has taken place largely, if not entirely, 

 by the loss of factors, it is more probable that the white race rustica 

 lost a factor which inhibited the formation of black pigment in the male, 

 than that the black race lost the power to produce black pigment. If 

 the latter alternative were true, the white insect produced would be in 

 the nature of an albino, and as is invariably the case with albinism in 

 other animals, it would be recessive to the coloured form. It has, how- 

 ever, been shown that the white race is almost completely dominant to 

 the black race, that is to say, it behaves as a "dominant white," like 

 the English rabbit. Now dominant whites are supposed to contain a 

 factor inhibiting pigment production ; and extracts from the skins of 

 English rabbits have been shown to contain an inhibitor capable of 

 preventing the production of pigment by the enzymes from the skins 

 of black and other coloured rabbits \ whereas extracts from the skins 

 of albinos fail to do so. Thus the suggestion that the dominant form 

 is phylogenetically the older is probably correct. 



Conclusions. 



1. It is contended that the evidence advanced by Cockayne is not 

 sufficient to deny segregation in the Mendelian sense, in crosses between 

 D. mendica and its white variety rustica. 



2. The experiments reported show that in the F^ generation the males 

 are not much deeper in colour than "pure rustica," so that this form 

 may be considered dominant to a great extent over the type; in fact, 

 some heterozygous insects are indistinguishable from mstica. 



3. The great variation in the buff coloured hybrids is probably due 

 to the effects of modifying genes. The colour of these buff insects as well 

 as that of the others was measured in the tintometer. The construction 



1 Onslow, H., Roy. Soc. Proc. B, Vol. lxxxix. p. 36, 1915. 



