DESCENT WITH CHANGE 



381 



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tion) from a population and then stops — though if selection is 



stopped the isolated lines usually merge soon again into the original 



population. A mutation must occur in the pure line for selection 



to be effective — but by _ 

 . ., . , Pure Line 



the mutation the single 



pure line becomes two. 



(Fig. 243.) 



Thus the pure line con- 

 cept has served to clarify 

 our ideas in regard to 

 selection by focussing at- 

 tention on the actual na- 

 ture of the variations be- 

 ing dealt with — to make 

 a sharp discrimination 

 between modifications, 

 which are a result of en- 

 vironmental influences 

 recurrent in each genera- 

 tion, and variations that 

 are heritable because they 5 



are the result of changes 

 in the germ plasm. 



However, it will be 



recognized at once that, 



in general, the animal 



breeder, as well as Nature, Population 



deals with hybrid stock, 



heterozygous in regard to 



many characters, rather _, _._ „. 



Fig. 243. — Diagram to illustrate a popula- 

 tion pure lines. rLven t } on f beans and its five component pure 



pure lines do not Stay pure lines. The beans are assorted according to 



-mutations occur. Here ***&*- Tubes containing beans of the same 



. weight are placed in the same vertical row. 



selection has ample ma- Se e Fig. 240. (From Walter, after Johannsen.) 



terial at its disposal so it 



can and does isolate new combinations and accumulate mutations 

 in the direction of selection. If it is carried on sufficiently long, 

 the extent of the change may be very great: a more or less steady 

 change in the direction of selection when mutations are available. 

 Although selection is not 'creative,' it is effective: the appre- 



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