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 sa}^, 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° b}^ 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) . In flies 

 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 finall}^ 

 the flies that emerge later cannotbe told from normal 

 flies. If the culture is kept well fed the change does 



