RECAPITULATION 215 



entirely white. The theory of sympathy or correla- 

 tion might apply here since the lower side of the head 

 was unpigmented, but from the small size of the 

 specimen and the amount of pigment on the lower 

 side, it seems to me most probable that if the 

 specimen had lived to be adult the upper side 

 would have developed pigment under the action 

 of light and the specimen would have become 

 ambicolorate. 



When we compare the results reached by the 

 mutationists with those obtained by the Mendelians 

 we find that they tend to two different conceptions 

 of the relation between the gametes and the organism 

 developed from them. The effect of a change in 

 the determinants of the gametes according to the 

 mutationists is evident in every part of the plant. 

 A factor in Mendelian experiments usually affects 

 only one organ or one part of the organism. The 

 factor for double hallux in fowls, for instance, may 

 coexist with single comb or rose comb. The general 

 impression produced on the mind by study of 

 Mendelian phenomena is that the organism is a 

 mosaic of which every element corresponds to a 

 separate element in the chromosomes. Thus we 

 know that what we call a single factor may cause the 

 whole plumage of a fowl to have the detached barbs, 

 which constitutes the Silky character, but we also 

 know that an animal may be piebald, strongly pig- 

 mented in one part and white or unpigmented in 

 another. So we find in these Flat-fish mutations 

 mosaic-like forms which evidently result from 

 mosaic-like factors in the gametes, or in the chromo- 

 somes of the gametes. 



Experimental evidence concerning the movement 



