castle: role of selection in evolution 377 



be shown that beans are as devoid of genetic variation in other 

 particulars as they are in seed size, which the argument assumes 

 to be true. Further, if various pure lines of beans have come 

 into existence by an evolutionary process (descent from a common 

 ancestor, with modification) it is evident that differences must 

 have arisen which did not originally exist. Suppose we grant 

 Johannsen's (unproved) contention that such differences arise 

 by mutation only. If they arise in this way (or in any other way 

 whatsoever), selection can isolate them, and if they are at all 

 frequent in occurrence, selection can be continuously effective 

 in producing racial changes. It would all come down then to a 

 question of how frequent mutations are in a particular case. 

 Johannsen concedes their occurrence even in beans. It may 

 well be that in some organisms they are commoner than in others 

 and that in beans they happen to be particularly infrequent. 



2. Johannsen's case has been further generalized to include 

 all self-fertilizing organisms, which are supposed to fall auto- 

 matically into pure lines (i.e., those devoid of genetic variation) 

 as regards all characters. This too requires proof, but has been 

 found to be a safe working hypothesis in the case of cereals, 

 tobacco, peas, and other economic crops, in the attempted im- 

 provement of which selection of fluctuations, unless preceded 

 by hybridization may be regarded as a waste of time, for the 

 reason that genetic variation is so rare under continuous self- 

 fertilization that the breeder will obtain variation much more 

 quickly by resorting to hybridization. 



3. Further, it has been argued that if cross fertilization alone 

 interferes with the automatic production of pure lines, then any 

 organism which dispenses with fertilization altogether, reproduc- 

 ing asexually, must ipso facto constitute a pure line. Jennings 

 sought to test out this conclusion by experiment. He selected 

 size variations in Paramecium which reproduces by fission, 

 with success in the case of mass cultures of unknown origin, 

 but without success in the case of cultures made from single 

 individuals. This was regarded as strong confirmation of the 

 pure line principle until Calkins and Gregory, repeating the 

 experiment on ex-conjugants, were unable to support it. Then 



