212 THE BREEDING OF' ANIMALS 



volved much exercise and the ability to live on a scant 

 supply of food at certain seasons. These conditions have 

 resulted in changing the form of the body. The neck 

 and jaws are larger and well muscled. The back is sharp, 

 legs longer and ribs flatter than in the improved forms. 

 If the young pigs of these unimproved swine are placed 

 under conditions where they are supplied with an abun- 

 dance of nutritious food, they approach somewhat the well- 

 rounded form of the improved type. In each of these 

 cases the environment has resulted in causing a distinct 

 variation from the parent form. This variation can be 

 easily observed, it can be measured. But whether this 

 environment has influenced in any way or how much it 

 has influenced the elemental carriers of heredity in the 

 germ, it is impossible to do more than conjecture. It 

 cannot be accurately measured. Biologists are generally 

 agreed that in a case of this kind the germ is not funda- 

 mentally changed. 



200. Influence of use and disuse in causing modifica- 

 tions. The constant use of an organ in the performance 

 of work will modify the organ in accordance with the work 

 performed. Any organ of the body that is not used may 

 atrophy. Changes resulting from excessive use of the 

 various parts of the body may be so extensive as to appear 

 almost as new characters. If modifications resulting 

 from food, climate or mutilations are transmitted, the 

 changes resulting from the constant use of an organ should 

 be transmitted with equal or greater force. 



Lamarck's theory of evolution was largely based upon 

 the supposed adaptation of the various organs of the 

 body to their environment, and that such adaptations 

 were readily transmitted. Thus the giraffe, forced by 

 conditions to feed upon the leaves of trees, gradually 



