VARIATION. 101 



As the climate of Massachusetts is not so mild and 

 uniform as that of Ireland, we must attribute the 

 changes observed in these cattle to the influence of 

 shelter during the winter, in connection with a better 

 supply of food. 



It is a well-known law of the organization that the 

 highest development of any particular organ, or set of 

 organs, can only be attained by their repeated and 

 systematic exercise. 



The athlete, as well as the horse in training for a 

 race, must perform an amount of work that taxes the 

 system severely, to secure that strength and develop- 

 ment of the muscular system that fit him for the 

 best exhibition of his powers. The highest mental 

 development can only be obtained by severe intellect- 

 ual effort. 



Mr. Darwin has shown that the proportional weight 

 of the wing-bones of wild-ducks is greater than in tame 

 varieties, while the proportional weight of the leg- 

 bones is greatest in the latter. 1 



The activity of the glandular system depends 

 largely upon the demands made upon it, in accordance 

 with the same principle. 



Dr. Carpenter, in his article on the " Varieties of 

 Mankind," says : " Another remarkable fact, relative 

 to the oxen of South America, is recorded by M. 

 Roulin. In Colombia the practice of milking cows 

 was laid aside, owing to the great extent of the farms 

 and other circumstances. In a few generations the 

 natural structure of the parts and the natural state of 

 the function have been restored, the secretion of milk 



1 " Animals and Plants under Domestication," vol. L, p. 345. 



