4."> 



THE AMERICAN NATURALIST 



[Vol. L 



be named. So far as I know, a marked extension of the 

 web between digits I and II has never been observed; 

 between II and III it is relatively rare; again, between 

 IY and V it has, so far as I know, not been noted in 

 mammals. 



(c) Variation is Not Indefinite and Multifarious.— The 

 case of syndactylism in poultry well illustrates the gen- 

 eral principle of the limitation of mutation to particular 

 narrow lines ; and this is commonly the case. Thus, Dr. 

 J. A. Harris has examined himself and with the assist- 

 ance of others over 1,000,000 bean seedlings, and while 

 extraordinary variations have been found, yet in the 

 later hundreds of thousands no new ones have appeared; 

 nevertheless, the possibilities in leaf form, variegation, 

 etc., if we may judge from books on plant teratology, are 

 by no means exhausted. And when we contemplate the 

 variety of form assumed by first leaves in different 

 species, is not the constancy in the form in beans striking 

 evidence of the narrowness of variation and its restric- 

 tion to certain lines? 



{d) Evolution by Loss of Genes.— Finally, it has long 

 been recognized that an extraordinarily large proportion 

 of the mutations we meet with are recessive to the wild 

 type. Castle has noted this of rabbits and guinea pigs 3 

 and Morgan has noted it in the case of Drosophila. In 

 my work with poultry I was not impressed with it, as 

 taillessness, polydactylism, syndactylism, white color of 

 the Leghorn, rose comb, etc., are dominant mutations. 

 Still, other genetics work (Baur, Shull and many others) 

 is strengthening the conclusion that current evolutionary 

 changes under observation are chiefly due to the dropping 

 out of genes, and this supports the theory that evolution 

 is proceeding largely by the loss of genes. However, the 

 fact of dominant mutations can not be derived. Take, 

 for instance, foot abnormality. Here a disturbing factor 

 has appeared in the organism that was not there before. 

 There is no a priori reason for doubting that the break- 

 s'' Heredity," p. 86. 



