380 castle: role of selection in evolution 



ently heritable. Variation in one or more of these factors (by 

 mutation or otherwise) would undoubtedly influence the total 

 productiveness, and the probability of the occurrence of a muta- 

 tion would increase with the number of factors involved. So 

 that even one formally committed to the pure line doctrine, but 

 admitting as Johannsen does that mutations do occasionally 

 occur in pure lines, might hopefully continue to look for improve- 

 ment in the standard of egg-production. No other method of 

 detecting and utilizing a favorable variation, when it does occur, 

 can be suggested than the very method of methodical and per- 

 sistent selection against which the pure line advocates direct 

 such vigorous attacks. 



Morgan is a formal adherent of the pure line doctrine, but 

 pragmatically a selectionist for he admits the great progress 

 made in the improvement of domestic animals and plants by 

 selection, and even that his own mutants of Drosophila fluctuate 

 and yield modified forms in response to methodical selection, 

 as for example the bar-eyed mutant, subjected with success to 

 plus and minus selection by Zeleny. But he attempts to explain 

 these results in harmony with the pure line principle by assum- 

 ing that, whenever a modification is observed in any character, 

 this is due to a mutation, and if a graded series of modifications 

 is obtained, as in the plus and minus selected bar-eyed Droso- 

 phila, this is due to a multiplicity of mutating factors whose 

 action on the chief factor concerned is purely incidental. On 

 this view, however, the attainment of a completely homozygous 

 condition on the part of all factors (if all are indeed Mendelian) 

 would put an end to genetic variability, and selection would 

 then cease to produce effects. Such a completely stable con- 

 dition has, however, rarely been demonstrated. One case is 

 reported by MacDowell, that of a race of Drosophila with an 

 extra number of thoracic bristles. The average number of 

 bristles was increased by selection for six generations but then 

 showed no further increase and could not subsequently be 

 changed either upward or downward by further selection. The 

 race had apparently become a "pure line" for bristle number. 



