1 62 MENDEL'S LAWS 



276. It is admitted, then, that two or more dissimilar 

 allelomorphs sometimes unite permanently to form compound 

 allelomorphs which are transmitted to descendants as units. In 

 the case quoted above, the component characters are, I believe, 

 supposed to have been more or less patent, and therefore the 

 Stanley presented an appearance different from any of the types 

 from which it was obtained by cross-breeding. But, if compound 

 allelomorphs in which each component unit is patent may occur, 

 it is not unreasonable to suppose that allelomorphs, supposing 

 they exist, may sometimes combine to form more or less permanent 

 compounds in which one (or more) of the components is patent 

 and the other (or others) latent. That this has happened at 

 least very frequently there can be no possible doubt. We see it 

 in aphides, in which the allelomorphs for the male characters are 

 latent all the summer. Every case of the reappearance of a latent 

 ancestral character (at any rate in a pure-bred race) l is an instance 

 of it. 



277. If we cross a Japanese waltzing mouse with an albino 

 mouse, then the ancestral wild-grey colour reappears in a large 

 proportion of the offspring. It is supposed, doubtless correctly, by 

 Mendelians, that the special characters of the two domestic breeds 

 arose originally as mutations from the wild type. Since it is very 

 improbable that two animals varied discontinuously in the same 

 way and at the same time and place, the mutants (as the individuals 

 that underwent mutation are termed) were crossed with the wild 

 type. 2 The reproduction was Mendelian, and artificial selection 

 established a race of ' pure ' recessives amongst the descendants 

 Japanese waltzing mice in the one case and albinos in the other. 

 According to the Mendelian doctrine of unit segregation and 

 gametic purity the grey colour should have been quite absent 

 (not latent) from both varieties. 3 It was only latent, however. 

 It follows that in this case there was no unit segregation nor 

 gametic purity. What really occurred was, not the elimination 

 of the ' grey allelomorph ' but the formation of a compound 



1 See 280. 



z " Mendel's discovery, it will be understood, applies only to the manner of trans- 

 mission of a character already existing. It makes no suggestion as to the manner 

 in which such a character came into existence. The facts, however, leave no room 

 for doubt that at least one character of each pair of simple allelomorphs has arisen 

 discontinuously. The fact that the gametes of the cross transmit each member 

 of the pair pure, is as strong an indication as can be desired of the discontinuity 

 between them." First Report to Evolution Committee, p. 151. See 290. 



8 See 255. 



