^""^ THE NEW EVOLUTION "^^^^ 



fishes, salps (fig. 58, p. iii), crustaceans, young 

 fishes, mollusks, arrow-worms (fig. 6i, p. iii) and 

 other creatures which live at or near the surface of 

 the sea. 



But we find no transparent creatures on the land. 

 All land mollusks and land crustaceans are opaque, 

 though some of their relatives in the sea are beauti- 

 fully transparent. The reason seems to be that all 

 land animals must of necessity rest upon an opaque 

 surface. They therefore must be protected against 

 light as it comes to them from the sky and also as it is 

 reflected in modified form from beneath and all about 

 them. Besides, on land transparency would not mean 

 invisibility, for the difference in the refractive index 

 between air and protoplasm is too great ever to be 

 overcome. So any transparent creature on the land 

 would be as readily visible as a glass model of itself. 

 But in the sea, especially if the light be dim, trans- 

 parency means from partial to almost complete 

 invisibility. 



The second means of protecting the animal body 

 against an excess of light is through the development 

 of pigment or coloring matter in the superficial body 

 covering or its outgrowths — in the skin or in hair, 

 feathers, plates, scales, or other structures. This 

 method is seen in all the animals which live exposed 

 on land, and in most of the animals of the sea. Even 

 the transparent marine creatures develop pigment 

 more or less extensively about such internal organs as 

 would be injured by too much light. 



Thus all land animals as well as most sea creatures 



