What is Selected 



353 



by selecting in the direction of the extreme. Later studies, 

 however, raised many doubts as to just what was accom- 

 pHshed by the selection. There is no doubt about the final 

 appearance of the animals. There is no doubt as to pheno- 



0(}8(J(S(5(5!59 



(50(5(50(3 839 



Fig. 92. Pure Lines in Paramecium 



In these one-celled animals reproduction is by cell division. By follow- 

 ing up the descendants of a single cell it is found that although in each 

 generation the progeny varies in size, the descendants of a large individual 

 furnish the same kind of population as the descendants of a small individual. 

 Each row represents one such pure line, 4" marking the mode. There is a 

 great deal of overlapping, so that in a mixed population of several such pure 

 lines the distribution about the mode, X, suggests the continuous variation 

 found in a single strain. Selection from such a mixed population may 

 isolate several genotypes, but apparently does not change the species. After 

 Jennings. 



types in the successive generations. The doubt is raised by 

 Morgan and others as to just what happens in the hereditary 

 mechanism. 



We have already seen that pigmentation and coat pat- 

 tern may be determined by many factors. Some of these 

 factors act as modifiers, some diluting, some intensify the 



