FUNCTIONAL VARIATION 95 



At what period the reproductive functions are most success- 

 ful in producing young of a high order of excellence, whether in 

 youth, middle age, or old age, is not determined, though it is a 

 point on which light is badly needed. It is certain that the 

 practice of using young and immature sires is almost universal, 

 especially among cattle. That there is danger in continued 

 reproduction from immature animals, even though they are sex- 

 ually vigorous, there is grave reason to fear, and yet, in general, 

 reproduction antedates maturity. 



The duration of profitable service depends, of course, upon 

 the nature of the function involved. The life service of a racing 

 horse is manifestly less than that of a work horse, and the life of 

 a meat animal is less than that of one kept for milk. 



Influence of exercise : use and disuse. The beneficial effect of 

 use in developing and perfecting the functions of the body has 

 been recognized from the most ancient times. Athletes train 

 for this purpose. Musicians practice for many hours every day ; 

 indeed their chief labors arise from the need of constant and 

 severe exercise of the musical faculties in order to achieve any 

 considerable degree of perfection and to hold it after it has 

 been acquired. 



Horses designed for racing are worked almost from the first 

 in order to make the most of any natural ability to trot or run. 

 Cows are believed to be more efficient producers of milk if they 

 begin at two years of age and are kept, so to speak, in constant 

 practice, and barrenness is believed to be less likely if heifers 

 are bred early than if left to attain maturity without produc- 

 ing young. 



Darwin discovered that the wing bones of wild ducks as com- 

 pared with their leg bones were relatively heavier than those of 

 tame ducks, corresponding to their respective habits of life. 

 The arm of the blacksmith and the wing of the ostrich ; the 

 remarkable leg of the kangaroo and the remains of that of the 

 whale ; the brain power of the busy man and that of the slug- 

 gard, these and scores of examples that might be cited show 

 not only that exercise develops, quickens, and perfects the body 

 functions, but they show, too, that their very retention or loss 

 depends in the long run upon their constant and rational use. 



