COLOR IN ANIMALS 117 



Signals and recognition marks ; 



Confusing coloration ; 



Sexual coloration. 



We are not interested, in this connection, in non- 

 useful colors, or in the direct physiological value of colors. 

 The other uses of color, however, present a diverse series 

 of phenomena very significant in the light of the theory 

 of evolution. 



Protective coloration and resemblances, 



\Ye referred in the early pages of this book to the severity 

 of the struggle for existence and to the importance of any 

 structure or character which enables its possessor to escape 

 destruction. Carnivorous animals are so common and so 

 voracious that, as we would naturally expect to find, their 

 prey have adopted various means of defence. Among these 

 some of the most important have to do with color. Ani- 

 mals which closely resemble their environment in color 

 will escape the notice of their enemies and thus be pre- 

 served, while their less protectively colored neighbors will 

 be seen, captured, and devoured. Natural selection will 

 thus tend to produce protective coloration. The principle 

 must be sufficientlv clear. Let us observe a number of 



j 



instances of such coloration. 



Many animals which live at the surface of the open 

 ocean are transparent, so as to be distinguished only with 

 difficulty from the water itself. This is true of many of 

 the jellyfishes and their relatives the ctenophores and 

 siphonophores, of most pelagic Crustacea and worms, of 

 the pelagic tunicates, and many other less familiar forms, 

 and of almost all marine larvae. This invisibility must be 



