254 



THE AMERICAN NATURALIST 



[Vol. L 



record its occurrence, a view I by no means share. But, it may 

 be asked, what control can we exercise over the event ? We can 

 prevent or permit it at will. For observation shows that if we 

 permit the individual to mate only with those of inferior value, 

 we shall get no offspring of the highest grade. Thus a 38 -f- a 3B 

 produces commonly only A 37 , rarely A 38 and, we might say, never 

 A 30 . But if we permit the individual to mate with individuals 

 of equally high grade (and this is what selection in a particular 

 direction does) experience shows that a majority of the offspring 

 will be of that same grade, but a few will be of higher grade. 

 These few make possible further advances. Thus a 39 makes pos- 

 sible the subsequent attainment of a 40 . Whether this relation- 

 ship involves "causation" or not is a question for the logicians 

 and methodologists, of whom I am not one. As to the fact our 

 rat experiments leave no doubt. In the light of such facts it 

 seems to me that a view earlier held among biologists, that vari- 

 ability is one of the fundamental properties of organisms, comes 

 nearer to the truth than this modern notion of the pure, unvary- 

 ing line. This pure-line concept Pearl rightly characterizes as 

 "one of the most useful working tools in the practical breeding 

 of plants and animals that has ever appeared." Why useful? 

 Because it has caused us to pause and take careful inventory of 

 our facts, and to discard as rubbish many loosely held notions. 

 But Pearl reminds us that not all the pure linist's facts are in one 

 basket with Johannsen's beans, nor even in that other vanished 

 basket with Jenning's paramecia. There are, he reminds us, 

 ' ' all the Svalof oats and wheats to be reckoned with. ' ' True and 

 they are mute witnesses to the cumulative effects of selection. 

 For all agree that these pure lines of oats and wheats are the 

 product of continuous self-fertilization. And what more than 

 self-fertilization renders possible generation after generation the 

 union of gamete with its like, the indispensable condition for pro- 

 gressive variation in a particular direction, as I have tried to 



Intelligent selection only accelerates this natural process of 

 progressive variation, for it singles out the individual which is 

 producing gametes of unusual value and permits the union of 

 such high grade gametes only with gametes of their own sort, so 

 that step after step in a particular direction becomes possible, 

 where unguided self-fertilization would give only halting and 

 uncertain progress. Can we doubt that it is progressive varia- 



