162 MENDELISM CHAP. 



which the varying conditions 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 10, 12, 14 grains 

 respectively, and if the range of fluctuation of each of 

 these pure lines is 1 2 grains, then our curve must be repre- 

 sented as made up of the three components 



A fluctuating between 4 and 16 with a mean of 10 

 B ,,6 18 12 



8 ,, 20 ,, ,, 14 



as is shown in Fig. 31. A seed that weighs 12 grains may 

 belong to any of these three strains. It may be an aver- 

 age 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 be 

 fluctuations of different pure lines. But within the pure 

 line any seed, large or small, produces the average result 

 for that line. Thus a seed of line C which' weighs 20 

 grains will give practically the same result as one that 

 weighs 10 grains. 



On this view we can understand why selection of the 

 largest seed raises the average weight in the next genera- 

 tion. We are picking out more of C and less of A and B, 

 and as this process is repeated the proportion of C gradu- 

 ally increases and we get the appearance of selection act- 



