FUNCTIONAL VARIATION 77 



the breeder, intent upon enhancing their service to man, sees 

 in the variation of the functions natural to domesticated animals 

 and plants the greatest opportunity for improvement. 



So true is all this that the successful breeder may be distin- 

 guished from the novice at this point. The latter is likely to be 

 attracted, first of all, by form or color, because differences of this 

 sort are striking and easily noticed ; while the former will always 

 keep foremost in mind the question, Why is this animal (or plant) 

 valuable to man, and what is it to do ? 



The student cannot, therefore, know too much about the 

 natural functions of domesticated animals and plants and the 

 deviations to which they are subject. He should know this, not 

 only as a guide to his selection but also as constituting valuable 

 information upon the nature of evolution and the possibility of 

 influencing the causes that control the development of living 

 beings, functionally as well as structurally. 



Instances of variation in functional activity are easily divisible 

 into four classes. 



1. Variation in the degree of activity of normal functions 

 between different individuals of the same species. 



2. Variations in the degree of activity of normal functions 

 within the same individual. 



3. Modification of normal functions by external or other 

 influences. 



4. Normal functions exercised under abnormal conditions. 



SECTION I VARIATION IN THE DEGREE OF ACTIVITY 



OF NORMAL FUNCTIONS BETWEEN DIFFERENT 



INDIVIDUALS OF THE SAME SPECIES 



Variation in milk secretion. This is a function peculiar to one 

 class of animals (mammals). It is the product of a highly spe- 

 cialized structure and is practically confined to the female sex. 

 Moreover, it is of peculiar economic as well as physiological im- 

 portance, and there is no better example to bring out much that 

 is involved in functional variation. 



The structure of mammary-gland tissue is characteristic 

 wherever found, but the quality and flavor of its product (milk) 



