THE PRACTICAL VALUE OF GENETICS 351 



Selection. The process of selection has been practiced for 

 many centuries for the production of improved strains of cultivated 

 plants and domestic animals. It is a logical consequence of the 

 fact that "like produces like" to a marked degree. Cows which 

 produce a large quantity of rich milk and bulls of the same strain 

 are much more likely to produce good dairy cattle than those 

 which possess other qualities. Sheep with fine and heavy fleeces 

 are obviously more valuable for the production of wool than 

 those whose fleeces are coarse and light. By the early recognition 

 of these facts man has produced beef and dairy cattle, draft and 

 race horses, dogs of many breeds and a multitude of other distinct 

 varieties of relatively few natural species (Fig. 195). 



Methods of Selection. Before the discovery of scientific 

 principles of inheritance selection was necessarily based upon 

 observed characters, and hence may be called phenotypic selection. 

 It is usually known as mass selection because the best individuals 

 from a given group of organisms are selected as the parents of 

 future generations and reproduction of the poorer individuals is 

 prevented, but no further attention is given to the details of 

 parentage. 



Closer attention to individual parentage brought about the 

 refinement of method known as line selection, which is closely 

 allied to the most modern and scientific method, genotypic selec- 

 tion. 



Mass Selection. The English scientist, Hallet, associated se- 

 lection with environmental effects by giving plants the best 

 possible environment and selecting those which did best under 

 these favorable conditions. Rimpau, on the other hand, subjected 

 his grains to unfavorable or merely average conditions, and 

 selected those which showed the ability to do well in spite of 

 adverse surroundings. Either method results in the improvement 

 of the organism, but the latter in particular is valuable for it 

 discloses something of the inherent possibilities of the individual. 



Selection as it has unavoidably been limited in the honey-bee 

 is a fine example of the effectiveness of mass selection. Since the 

 functional sexes are merely reproductive and the individuals 

 which are directly of use to man do not reproduce, the breeding 

 stock in this case can be judged only by its progeny. The mating 

 of bees occurs in flight so that only the female parentage of a 

 colony can be definitely known and selected. Only within the 



