TYPES OF MENDELIAN HEREDITY 47 
upon the fact that whenever (as often happens) all 
other conditions, external and internal, that modify 
characters remain constant, clear-cut ratios are ob- 
tained which can be explained only as due to segre- 
gation, in definite ways, of particular hereditary 
factors that perpetuate themselves unchanged from 
generation to generation. The validity of the fac- 
torial hypothesis may also be proved under circum- 
stances not so well controlled, however. In cases 
where, on the factorial hypothesis, a certain factor 
is expected to be present in an individual, then, 
even if the individual fails to develop the character 
commonly taken as indicative of the factor, the actual 
presence of the factor may be demonstrated by breed- 
ing tests. For if, in subsequent generations, cir- 
cumstances—genetic or environmental—are provided, 
like those in which the character previously appeared, 
it will again show itself. Flies of the race with ab- 
normal abdomen, if raised in a dry bottle, appear 
perfectly normal, but the presence within them of the 
factor for abnormal may be demonstrated by rear- 
ing their offspring in a wet bottle. Again, the factor 
for pink eyes may be carried by a race with white 
eyes, and although pink does not show in the white- 
eyed race, its presence there may then be demon- 
strated by crosses of these flies with flies that are not 
white. Cases like these could be multiplied over and 
over again. 
