ACTION OF FORCES ON ORGANISMS. 49 



colours and useless corrugations and excrescences, 

 and in reproducing such a number of useless rudi- 

 mentary organs. The plain facts lead to the con- 

 clusion that pigment is caused by light acting upon 

 the tissues, and where there is no light there can 

 be no pigment. 



In the case of the rooster described by Darwin, 

 which was reared in the dark, and never crowed, 

 but grew up with the instincts of a hen, we have 

 an instance of arrested and distorted development 

 which shows how extremely important the light is for 

 the proper development of an animal whose race has 

 always lived in the light. Professor Yung's experi- 

 ments showed not only the importance of light, but 

 that different kinds of light have a distinctly differ- 

 ent effect upon the development of an organism. 

 Ordinary white light, as shown by Professor Yung, 

 is most favourable to the normal development. 



Of the light that falls on an animal, a part is 

 absorbed or reflected by the pigment in the skin, 

 and a part is transmitted through the semilucent 

 tissues beneath. It is this latter portion of light 

 which penetrates the body, which we would natu- 

 rally expect to influence the organism's vital func- 

 tions. Other things being equal, a light coloured 

 or white skin admits more light, and a dark or 

 black skin less light. In nature, therefore, the 



