castle: r6le of selection in evolution 379 



but this is simply juggling with names, giving a new meaning 

 to the word mutation in order to justify a sweeping generaliza- 

 tion otherwise untenable. 



The test of a pure line is its freedom from any genetic varia- 

 tion, so that selection cannot modify the racial mean as regards 

 any character. As soon as any race of animals or plants changes 

 in response to selection, it must be forthwith excluded from the 

 category of pure lines. The consequence is that no case of a 

 pure line among animals has yet been demonstrated. Never- 

 theless the "principle of the pure line" is in some way or other 

 supposed by the followers of Johannsen to confer on even the 

 higher animals a limited liability to modification in consequence 

 of selection. 



Thus Pearl having been entrusted in 1908 with a selection 

 experiment for increase of egg production in Plymouth Rock 

 fowls, an experiment which had already been in progress for 

 nine years, decided after a study of the records kept by his 

 predecessor that no improvement whatever had up to that time 

 been made and further that none probably could be made since 

 individual wild birds probably lay, under favorable conditions, 

 as many eggs as their best tame relatives. This reasoning was 

 strictly in accordance with the "pure line principle" and was in 

 fact based on it. 



Later by changing somewhat the basis of selection, so as to 

 rank, his animals on the basis of their progenies' performance as 

 well as their own, Pearl found that he could considerably in- 

 crease the flock average. Yet he still maintains that he has 

 only more good birds not better ones, than when the experiment 

 began, and in loyalty to the pure line principle he has no expecta- 

 tion of .obtaining better ones in the future, since he already has 

 and has had all along the ne plus ultra sort. One less devoted 

 than Pearl to a generalization of the pure line doctrine would 

 continue hopefully the effort to produce a better fowl as well as 

 to produce more good ones. For the function of egg-production 

 admittedly depends upon many physiological factors (as well as 

 several external ones). These physiological factors must many 

 of them be independently variable and to some extent independ- 



