400 EVOLUTION AXD ANIMAL LIFE 



presence or absence of light, by virtue of their capacity to 

 absorb hght and thus stimulate the specially sensitive cells 

 composing them. And the pigment or absence of it (dark 

 or light color) in the fur and plumage of certain mammals and 

 birds may perhaps serve to absorb or to reflect the sun's rays 

 so as to help keep warm or cool the animals thus colored. But 

 such explanations of animal colors can obviously apply to but 

 few cases. Very plainly color, and especially pattern, has its 

 significance if anywhere in connection with certain special re- 

 lations of animals to other animals and to the world generally. 

 So, ever since the days of Darwin, two general categories 

 of such significance or explanation of color and pattern have 

 been in the minds of naturalists. One of these is the signifi- 

 cance attributed to color pattern by the theory of sexual 

 selection; the other is that attributed to it by the general theory 

 or group of theories of protective resemblance, recognition, 

 warning, directive, and mimetic coloration, etc. Of these two 

 general explanations, one has steadily lost ground since Dar- 

 winian and early post-Darwinian days, while the other has 

 slowly but steadily gained adherents and has been extended to 

 cover more and more cases of animal ornamentation. Of the 

 theory of sexual selection it must be said that it certainly can- 

 not explain the conditions of secondary sexual differences, 

 including colors and patterns, in many groups of animals, and 

 it has really not been proved to explain them in any single 

 group, although in the case of birds and mammals it seems 

 possible that the theory is applicable: at least no other ex- 

 planation of equal validity has yet been presented. Of the 

 specialization of color and pattern for the sake of protecting 

 the animal by making it so harmonize or fuse with the usual 

 environment as to be indistinguishable, or by making it simulate 

 with sufficient fidelity some particular part of its surroundings 

 as a green or dead leaf, a twig, the dropping of a bird, a bit of 

 lichen or what not, or by making it mimic some other animal 

 notoriously well defended by sting or fangs or ill-tasting body, 

 so that the otherwise defenseless mimicker is mistaken by its 

 enemies for the defended mimicked kind of animal — of this 

 specialization and utility of color and pattern, evidence for its 

 reality is gradually accumulating to convincing amount. And 

 it is of this sort of color and pattern specialization that the 

 brief discussion to follow will be devoted. 



