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THE AMERICAN NATURALIST [Vol. L 



the ease of most characters for which our domestic animals are 

 valued and as regards which their improvement has been at- 

 tempted by selection. In such cases there has been a series of 

 slight advances, and everything indicates that the order of the 

 advances is significant and necessary, that the higher stages can 

 be attained only by passing through the lower ones. If this is so, 

 we need not quibble about "causation," but we may assure our- 

 selves that if we wish to attain a distant goal, the first thing to do 

 is to make for intermediate points. 



I regard it as a hopeful sign that Pearl can see no reason why 

 genetic changes may not be small in amount in some cases, even 

 though large in others. This I hope is only a first step toward 

 the complete abandonment of that "real, genuine pure-line body 

 of doctrine" which he still holds dear. 



W. E. Castle 



Bussey Institution, 

 Forest Hills, Mass., 

 February 16, 1916 



