132 EVOLUTION AND GENETICS 



a new generation from one plant or indeed from 

 several plants of any one of these lines. Each line 

 repeats the same modal class. There is no further 

 breaking up into groups. Within the line it does not 

 matter at all whether one chooses a big bean or a 

 little one — they will give the same result. In a word, 

 the germ-material in each of these lines is pure, or 

 homozygous, as we say. The differences that are 

 found between the weights (or sizes) of the indi- 

 vidual beans are due to their location in the pod or 

 in the plant on which they have developed. 



Johannsen's work shows that the frequency dis- 

 tribution of a pure line is due to factors that are ex- 

 trinsic to the germ-jjlasm. It does not matter then 

 which individuals in a pure line are used to breed 

 from, for they all carry the same germ-material. 



We can now understand more clearly how selec- 

 tion acting on a general population brings about, at 

 first, changes in the direction of selection. 



An individual is picked out from the population 

 in order to test its particular kind of germ-material. 

 Although the different classes of individuals may 

 overlap, so that one can not always judge an indi- 

 vidual from its appearance, nevertheless, on the 

 whole, chance favors the picking out of the kind of 

 genn-material sought. In species with separate 

 sexes there is the further difficulty that two indi- 

 viduals must be chosen for each mating, and super- 

 ficial examination of them does not insure that they 



