38 TYPES OF MENDELIAN HEREDITY 
angles to the body. In the case of rudimentary and 
truncate (Fig. 18) the wings are so similar that 
without breeding tests one of them might easily be 
taken for the other. Finally, ‘facet’”’ and “rough” 
both have the ommatidia of the eye disarranged very 
much in the same way. 
MopIFIcATION OF THE EFFECTS OF FAcToRS 
I. By Environmental Influences 
It is a commonplace that the environment is es- 
sential for the development of any trait, and that 
traits may differ according to the environment in 
which they develop. In most cases different genetic 
types produce different results in any ordinary 
environment. The environment, being common to 
the two, may therefore in such cases be ignored, 
or rather taken for granted. There are other cases, 
however, in which a particular genetic type appears 
different from another one only in a special environ- 
ment. Where this environment is not the normal 
one, its discovery is an essential element of the 
experiment. 
One of the best cases is that given by Baur. The 
red primrose (Primula sinensis rubra) reared at a tem- 
perature of 30°-35° C. (with moisture and shade) 
has pure white flowers, but the same plants reared at 
15°-20° have red flowers. If the white-bearing plants 
are brought into a cooler place, the flowers that are 
already in bloom remain white, but those that de- 
velop later in the cooler temperature are red. There 
