No. 534] 



THE PURE LINE THEORY 



351 



tion that selection within the pure line is ineffective. 12 

 The strenuousness with which this has been maintained 

 has even engendered in some minds the opinion that se- 

 lection has no role at all to play in evolution or in prac- 

 tical breeding. The attitude of many appears to be that 

 Darwin was quite mistaken when he wrote, 1 1 The key is 

 man's power of accumulative selection: nature gives suc- 

 cessive variations; man adds them up in certain direc- 

 tions useful to him." 



Darwin "said, "If selection consisted merely in sepa- 

 rating some very distinct variety, and breeding from it, 

 the principle would be so obvious as hardly to be worth 

 notice." Fifty years after this was written we hold a 

 symposium to celebrate the discovery that selection is 

 after all merely the isolation of distinct varieties ! 



Was Darwin right or wrong? Have all practical 

 breeders except those at the oft-quoted Svalof station 

 been chiefly occupied in wasting their time for the last 

 fifty years ? These are very important questions. 



The burden of proof obviously lies on the genotypists. 13 

 Much of the evidence offered is most general and not at 

 all unzweideutig. Indeed, when closely analyzed much of 

 the reasoning reduces to a circle of three arcs each of 

 one hundred and twenty degrees : 



1. Definition.— A genotype or biotype is an organic 

 unit, reproducing itself constantly 14 except for the transi- 

 tory, non-inheritable modifications due to environmental 

 influence. 15 It is not capable of change by selection. 



