742 



THE AMERICAN NATURALIST . [Vol. L 



hooded rats. The occurrence of this ''mutant" is 

 claimed to have been induced by selection. 



It seems to us quite improbable that this plus mutation could have 

 arisen in the minus selection scries. We believe that the repeated se- 

 lection which was practiced had something to do with inducing this 

 change in the plus direction (Castle and Phillips, :14, p. 31). 



No reason for such a supposition is given. On the 

 other hand there is clear reason for supposing that such a 

 mutation would be far more easily detected in the plus 

 series if it occurred there. The same mutation occurring 

 in the minus race would perhaps have the same relation 

 to that race as it had to the plus race when it occurred 

 there; since it would lack the extension factors of the 

 plus race, it would have a very different appearance and 

 would probably have a grade not far from "0." It seems 

 that very few rats of this grade were bred or tested. Had 

 this mutation occurred in the minus race and been iso- 

 lated, it would have been possible to obtain it as it ap- 

 peared in the plus race, by proper crossing. 



:12a. The Inconstancy of Unit -Characters. Amer. Nat., Vol. 46, p. 

 :126. Some Biological Principles of Animal Breeding. Amer. Breed. 



15a. Mr. Muller on The Constancy of Mendelian Factors. Ames. Nat., 

 Vol. 29, p. 37. 



:15b. Some Experiments in Mass Selection. Amer. Nat., Vol. 49, 

 p. 713. 



.16. Is Selection the More Important Agency in Evolution? Sci. Mo., 

 Vol. 2, p. 91. 

 Castle, W. E. and Phillips, J. C. 



:14. Piebald Eats and Selection. Pub. Cam. Inst. Wash. No. 195. 

 Hagedoorn, A. L. and A. C, :14. 



:14. Studies on Variation and Selection. Zt. f. ind. Abs. u. Vereroungs- 

 lehre, Bd. II, Heft 3, pp. 145-183. 

 MacDowell, E. C. 



:15. Bristle Inheritance in Brosophila. Journ. Exp. Zool., Vol. 19, 

 p. 61. 



