192 



MENDELISM 



own average weight about which the varying con- 

 ditions of the environment cause it to fluctuate. 

 Each of these strains is termed a pure line. If we 

 imagine that there are three such pure lines in our 

 imaginary case, with average weights lo, 12, 14 

 grains respectively, and if the range of fluctuation 

 of each of these pure lines is 1 2 grains, then our 



4 6 8 10 12 14 16 18 20 

 Weight of individual seeds 



Fig. 48. 



Curves to illustrate the conception of pure lines in a population. 



curve must be represented as made up of the three 

 components 



A fluctuating between 4 and 16 with a mean of 10 

 ^ I! » 6 „ 18 „ „ 12 



C " „ 8 „ 20 „ „ 14 ^ 



as is shown in Fig. 48. A seed that weighs 12 

 grains may belong to any of these three strains. It 

 may be an average seed of B, or a rather large seed' 

 of A, or a rather small seed of C. If it belongs to 

 B its offspring will average 12 grains, if to A 

 they will average 10 grains, and if to C they 

 will average 14 grains. Seeds of similar weight 

 may give a different result because they happen to 



