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- 
Fra. 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 
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