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, Morgan carried out a selection experi- 
ment for three years, involving upward of 75 gener- 
ations. The character selected was a dark “trident”’ 
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 mu- 
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 yseadily 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 
