VARIATION 



17 



certain details of structure. For example, note the prominent milk veins 

 on the udder of Tilly Alcartra as shown in Fig. 231. 



Psychological variations are differences in mental traits. That mental 

 and nervous conditions have very definite effects upon physical con- 



FIG. 1. Morphological variation in number, form and size of leaflets in the blue elderberry, 



Sambucus glauca. 



ditions is well known, but the problem of distinguishing between pur- 

 poseful action and automatic response, between manifestations of reason 

 and manifestations of instinct, is set for the students of animal behavior. 

 While variations in mental characteristics have an important place in 



eugenics and merit the attention of livestock breeders, yet the inheritance 

 2 



