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. 



MODIFICATION OF THE EFFECTS OF FACTORS 

 /. 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 



