200 DEVELOPMENT AND HEREDITY. 



is an interesting example of degeneration resulting 

 from a withdrawal of a single one of the growth- 

 stimulating forces. These animals, including fishes, 

 amphibians, crustaceans, and insects, are descended 

 from animals which lived above the surface of the 

 earth and possessed eyes. In the blind descendants 

 the eyes are present, but imperfectly developed. 

 There can be, I think, only one explanation of this 

 blindness, namely, that the eyes of many successive 

 generations have never been stimulated to their 

 full growth by light. If we admit the influence 

 of any force of the environment upon the develop- 

 ment of individual and race, then we must believe 

 that the action of light as a directing force was 

 of prime importance to the development of the eye. 

 It is a force which has always acted upon organisms 

 wherever eyes have been developed, and the eyes 

 have always stood in a direct relation to this force. 

 To suppose that eyes would develop and grow 

 equally well in darkness, is to accept a doctrine 

 of effects without causes, and causes without effects. 

 The theory has been advanced that the degeneration 

 of the eyes of cave-dwelling animals is due to natural 

 selection. It is supposed that the energy which 

 would be used in perfecting the eyes (which would 

 be useless in darkness) is saved in these blind 

 animals for other purposes. Therefore those with 



