TYPES OF MENDELIAN HEREDITY 39 
is another race of primula (Primula sinensis alba) 
that always has white flowers, even at 20°. Strictly 
speaking, we should say, not as we generally do for 
brevity’s sake, that the difference between the 
two races is that one has white, the other red flowers, 
but we should say rather that P. rubra reacts at 20° 
by producing red, at 30° by forming white flowers; 
P. alba, on the other hand, reacts both at 20° and at 
30° by producing white flowers. The constant dif- 
ference between these races is not in their color, but in 
the possibility of producing specific colors at certain 
temperatures. 
This is the point of view, of course, that must also 
be taken for cases in which differences exist in all the 
usual environments; for, here also, it is the different 
possibilities of reaction that are inherited. Brevity 
warrants us in speaking of particular characters as 
inherited, rather than the specific possibility of reac- 
tion that gave these characters; but no one need be 
misled by the shorter expression. 
Two similar cases of the influence of the environ- 
ment have been found in Drosophila. There is a 
mutant stock known as abnormal abdomen in which 
the normal black bands of the abdomen are broken 
and irregular or even entirely absent (Fig.19). Inflies 
reared on moist food the abnormality is extreme; 
but even in the same culture the flies that continue 
to hatch become less and less abnormal as the culture 
becomes more dry and the food scarce, until finally 
the flies that emerge later can notbe told from normal 
flies. If the culture is kept well fed the change does 
