264 THE HISTORY OF CREATION. 



be able to escape more easily, if their colour is as little 

 different as possible from that of their surroundings. If 

 therefore originally an animal species varied so as to present 

 cases of all colours, those individuals whose colour most 

 resembled the surroundings must have been most favoured 

 in the struggle for life. They remained more unobserved, 

 maintained and propagated themselves, while those 

 individuals or varieties differently coloured died out. 



I have tried to explain, by the same sympathetic selection 

 of colour, the wonderful fact that the majority of pelagic 

 animals — that is, of those which live on the surface of the 

 open sea — are bluish, or completely colourless and trans- 

 parent, hke glass and water itself Such colourless, glassy 

 animals are met with in the most different classes. To them 

 belong, among fish, the Helmicthyidse, through whose 

 crystalline bodies the words of a book can be read ; among 

 the molluscs, the finned snails (Heteropods) and sea- butter- 

 flies, or whales-food (Pteropods) ; among worms, the Salpse, 

 Alciope, and Sagitta ; further, a great number of pelagic 

 crabs (Crustacea), and the greater part of the Medusae 

 Umbrella-jellies, (Discomedusae) ; Comb-jellies, (Ctenophora). 

 All of these pelagic animals, which float on the sui'face of 

 the ocean, are transparent and colourless, like glass and like 

 the water itself, while their nearest kin live at the bottom of 

 the ocean, and are coloured and opaque like the inhabitants 

 of the land. This remarkable fact, like the sympathetic 

 colouring of the inhabitants of the earth, can be ex- 

 plained by natural selection. Among the ancestors of the 

 pelagic glass-like animals which showed a different degree of 

 colourlessness and transparency, those that were the most 

 colourless and transparent must have been most favoured 



