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, like glass and water itself. Such colourless, glassy 
animals are met with in the most different classes. To them 
belong, among fish, the Helmicthyide, 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 Salpz, 
Alciope, and Sagitta ; further, a great number of pelagic 
crabs (Crustacea), and the greater part of the Medusze 
Umbrella-jellies, (Discomedusze) ; Comb-jellies, (Ctenophora). 
All of these pelagic animals, which float on the surface 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, ike 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 
