Payne: Variations aiid Selection 



35 



experience, cultivated plants are not improved by selection 

 but by hybridization and mutation. These discoveries and 

 discussions have caused biologists in general to take an invoice 

 of their stock in trade. It has caused them to question 

 whether the small fluctuating variations of Darwin are of suffi- 

 cient advantage to the individuals possessing them to cause 

 such individuals to be preserved while neighboring individuals 

 not possessing such minute imperceptible variations are 

 destroyed. The more detailed study of heredity has also 

 caused most biologists to believe that these fluctuating varia- 

 tions are in the main merely somatic variations and hence not 

 inherited. In fact, Pearl ('17) goes so far as to say that 

 "The differences upon which natural selection directly oper- 

 ates are somatic differences by hypothesis and in fact." 

 Whether this is the exact meaning which Darwin meant to 

 convey is difficult to say from his writings. He realized 

 clearly that all fluctuating variations are not inherited, as in 

 the Origin of Species he says, ''These individual differences 

 are of the highest importance for us, for they are often 

 inherited." Again, however, he makes it very emphatic that 

 ''Under the term of 'variations' it must not be forgotten that 

 mere individual differences are included." He does not dis- 

 cuss why some individual differences are inherited and others 

 not, nor did he seem to realize clearly that the moment such 

 a difference is introduced, the two things would no longer be 

 the same. 



Since these doubtful points concerning the action of 

 natural selection have arisen, several experiments on the 

 effect of artificial selection have been designed and carried 

 out. It is not necessary to review these papers in detail, but 

 a brief sunnnary is desirable to lead up to a discussion of 

 the two main interpretations of the effect of selection. The 

 experiments have been made on two different types of mater- 

 ial. Some have been patterned after Johannsen's, i.e. to test 

 the effect of selection in pure lines. Others have worked on 

 bisexual forms. The conclusions reached from these recent 

 investigations depend in no way upon the material used. It 

 has been said by some and may be said by others that selec- 

 tion in bisexual forms is beset with difficulties and is not 

 comparable to selection within a pure line. This is in part 

 true. Castle maintains, I believe, that a bisexual strain which 

 is homozygous for any particular character is a pure line 



