52 DEVELOPMENT AND HEREDITY. 



all the colours of the spectrum in their proper 

 sequence, running from red on the head to violet 

 on the tail. This case is so remarkable that it 

 alone seems to prove that the colouration of ani- 

 mals is dependent on some peculiar reaction of the 

 physiological forces toward light, and not solely on 

 the protection or ornamentation of animals. 



Every organism which is capable of motion is 

 influenced in its growth by the external resistance 

 which is offered to its bodily motions, thus chang- 

 ing the strain and pressure upon the various parts. 

 The ordinary exercise of its organs which a young 

 animal, stimulated by the forces of its environment, 

 performs during the period of its post-natal develop- 

 ment, we know to be of great importance for the 

 proper growth of the animal. By exercise of the 

 legs and wings the nervous co-ordinations are per- 

 fected which control the walking and flying. This 

 principle is recognised and acted upon by the train- 

 ers of racing colts. Not only are the nervous co- 

 ordinations perfected in this way, but the full 

 growth of bone and muscle, and of the other organs 

 of the body, are dependent upon exercise. The 

 effects of exercise in the training for athletic con- 

 tests and gymnastic feats are so well known that 

 it is hardly necessary to mention them. What I 

 wish to emphasise is the influence of the ordinary 



