VARIATION 213 



extended his neck, which became longer, and this increase 

 in length was transmitted from generation to generation. 

 Wading birds, feeding in the shallow water along the shore, 

 gradually waded deeper and deeper into the water. 

 Their legs became longer, and the additional length gained 

 by each generation was transmitted. The long tongue 

 of the ant-eater, of woodpeckers, and humming birds, 

 was developed in a similar manner. The rudimentary 

 eyes of subterranean animals and fish in caves is another 

 supposed example of the loss of an organ through disuse. 



Among domestic animals, there are numerous examples 

 of a high degree of development of organs through con- 

 tinued exercise. The milking function in the dairy cow 

 can undoubtedly be greatly improved in any individual 

 by skillful exercise and use. The training of running and 

 trotting horses has resulted in very greatly increasing 

 the ability of an animal in those particular types of speed. 

 Are these modifications, the result of use or disuse, trans- 

 mitted by heredity? Such inheritance would be of the 

 very greatest importance to the . breeder of domestic 

 animals. In answer to this question no direct proof has 

 been offered that characters acquired by exercise or lost 

 by disuse are actually transmitted by heredity. 



201. Importance of causes of variation to the breeder 

 of domestic animals. While the researches of biologists 

 have led them to believe that the germ-plasm is very 

 stable and its character not easily changed by the environ- 

 ment of the body, it is nevertheless true that breeders of 

 the domestic animals have long believed that the amount 

 and kind of food, climate and training which animals 

 receive has an influence not only upon the individuals 

 benefiting by or suffering from such environment, but 

 likewise may have a profound influence upon their pos- 



