HEREDITY IN PURE LINES 215 



the environment and were not inherited, and that what was really in- 

 herited was merely a potentiality to produce beans of a certain average 

 weight which may be modified by the environment. It was also ob- 

 vious that these environmental modifications had no effect on subse- 

 quent generations, that is to say, they are not inherited. Selection, 

 therefore, has no effect unless there are hereditary differences among 

 the individuals selected. 



All the members of a given pure line are identical genetically, that 

 is, there are no differences in their genes. Johannsen called a group of 

 individuals that are alike in their genes a genotype, and spoke of all 

 such individuals, whether alike in seed size or not, as genotypically 

 identical. Hence individuals that breed alike, whether they look alike 

 or not, are members of the same genotype. On the other hand, each 

 of the nineteen pure lines is a different genotype though some beans 

 in each pure line may be exactly of the same size as some beans in 

 others. Johannsen designated individuals that look alike and have 

 the same somatic characteristics, whether they are genotypically 

 alike or not, members of the same phenotype; in other words, they may 

 be phenotypically identical though genotypically different. Phenotypic 

 differences, unless also associated with genotypic differences, are not 

 hereditary. This conclusion is a highly important one for all our fur- 

 ther study of heredity. 



Another significant conclusion may be reached from these experi- 

 ments, namely, that environmentally produced differences, if they are 

 merely phenotypic in character, play no important part in evolution, 

 except in so far as they may favor the survival of certain individuals 

 possessing genotypic differences. Hence, environmentally produced 

 differences in the soma have only an indirect influence on the course of 

 evolution. 



OTHER EXAMPLES OF PURE LINES 



W. L. Tower, in a long series of experiments on the potato beetle, 

 Leptinotarsa decemlineata, came to similar conclusions. He produced a 

 pure line, not by making use of a self-fertilizing species, but by closely 

 inbreeding a bisexual family for a long time. This stock was tested 

 and found to be genotypically pure, yet there was a considerable 

 amount of variability in the shade of color in the color patterns, and 

 in other ways. For twelve generations he selected and bred from the 

 darkest specimens, and from the lightest specimens, keeping the two 

 series separate (Fig. 51). The result was that, instead of getting 



