!()() THEORY OF EVOLUTION 



selection acting on a general population brings 

 about results in the direction of selection. 



An individual is picked out from the popu- 

 lation in order to get a particular kind of germ 

 j^lasm. Although the different classes of indi- 

 viduals may overlaj), so that one can not always 

 judge an individual from its appearance, never- 

 theless on the whole chance favors the picking 

 out of the kind of germ plasm sought. 



In sjDccies with separate sexes there is the 

 further difficulty that two individuals must be 

 chosen for each mating, and superficial examin- 

 ation of them does not insure that the}^ belong 

 to the same group — their germ plasm cannot 

 be inspected. Hence selection of biparental 

 forms is a precarious process, now going for- 

 ward, now backwards, now standing still. In 

 time, however, the process forward is almost 

 certain to take place if the selection is from 

 a heterogeneous population. Johannsen's 

 work was simplified because he started with 

 pure lines. In fact, had he not done so his 

 work would not have been essentially different 

 from that of any selection experiment of a pure 

 race of animals or plants. Whet^ier Johannsen 



