114 THE AMERICAN NATURALIST [Vol. LI 



ject to continuous genetic fluctuation. I have been unable to 

 produce or to discover any race of spotted animals which is free 

 from genetic fluctuation, though I have made an extended search. 

 If MacDowell or any one else has discovered such a race, let it 

 be produced. 



It does not, of course, follow that because white spotting in 

 rats is capable of indefinite modification through selection, there- 

 fore all heritable characters are equally capable of modification. 

 Physiological limitations no doubt often limit the modifiability of 

 characters. A sugar-beet can not be produced which is all sugar 

 or much over 25 per cent, sugar. There has to be retained a 

 plant mechanism for the production of sugar, a beet. Neither 

 is it to be expected that the thorax of Drosophila can be deco- 

 rated with an indefinite number of extra bristles. The bristles 

 have to be attached to something, and the thorax of Drosophila 

 is finite in size. It is not necessary to suppose that hypothetical 

 modifying factors have been used up simply because variation 

 ceases to progress in a particular direction. For no one, I sup- 

 pose, would contend that variation is equally easy in all direc- 

 tions and in all characters. De Vries has taught us the signifi- 

 cance of one-sided variation and we have become familiar with 

 recurrent types of variation which are encountered first in one 

 species and then in another. Such cases show that different 

 kinds of germplasm are similar in structure and likely to under- 

 go similar changes. But what happens to these spontaneous 

 variations when once they have put in an appearance depends 

 on external agencies, man or other factors in the struggle for 

 existence. The modern study of evolution has indeed empha- 

 sized the importance of spontaneous internal changes in pro- 

 ducing variations, but we still have to reckon with selection, nat- 

 ural and artificial, in determining the survival of variations as 

 well as in controlling their magnitude and the direction of their 

 further variation. 



W. E. Castle 



BussEY Institution, 



Forest Hills, Mass., 

 December 20, 1916 



