MULTIPLE FACTORS 259 



at any time and will be quickly observed if they are 

 in the direction in which a selective process is being 

 carried out. It may not be easy to recognize the 

 first appearance of a mutant and, in fact, its pres- 

 ence may be detected only after the selection has 

 gone so far that its origin is lost. The breeder may, 

 if he is not extremely observant, infer that his 

 selection is producing the desired effect on the 

 potency of the character, while in reality he is 

 studying the influence of a new factor on the charac- 

 ter under selection. This possibility may be il- 

 lustrated by two cases. In Castle's experiments 

 two rats appeared that behaved like a new type. 

 In fact he gives them the value of mutants. In 

 Drosophila, ]\Iorgan carried out a selection experi- 

 ment for three years, involving upward of 75 gener- 

 ations. The character selected was a dark 'Hrident" 

 on the thorax (Fig. 63). In a few generations a 

 minus stock with no trident was established that 

 bred true. The plus stock went up and down, the 

 selection being not always thorough. A stock that 

 always had the trident present to some degree was 

 obtained after a time. Later several other nui- 

 tations appeared, some of which greatly increased 

 the black on the thorax; some even swamped the 

 trident, making it a broad band. Three such mu- 

 tant stocks were readily isolated. It might have 

 been concluded that these mutations had occurred 

 in the direction of selection, because selection had 

 changed the potency of the trident factor, were it 

 not that during these three years over 100 other 



