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 Jan.,1913 CONCEALING AND RIVIALING COLORATION OF ANIMALS 13 patches of sky showing through, but much more often against a solid dark back- ground of canyon wall, river bank or solid green foliage. Of course this proves nothing, unless it coincides with the observations of others made with this partic- ular question in mind. Even in a level, treeless region, during the past season,  could not see that the low bounds of a cottontail ever carried its rump- high enough to bring it to the sky line, from the standpoint of a coyote, except for a fraction of a second at a time. Furthermore, the chief enemies of the rabbit, in many localities, are not mammals, but large birds of prey, which surely do not often see their quarry against white skies. This is also true of the white-tailed prairie-dogs of the intermountain region of Colorado. Whatever their purpose, that such white posterior markings are obliterative from the standpoint of the possessor's enemies is' altogether too doubtful to be accepted without thorough testing on many species. Mr. Thayer's photographs do not show the animals in the position in which they would usually be seen by their enemies at the "critical moment," in my judgment. Thayer's theory of countershading seems correct as an optical principle, but needs to be studied from many angles before acceptance as a part of the conceal- ing coloration theory. Many animals which are rendered quite indistinct in the middle of the day, when the light comes from above, are not so indistinct earlier and later in the day. Thayer himself admits this, apparently, in the case of the jacanas. I have found it true with the cottontails of the western plains during the past summer. As many countershaded animals are in hiding during the hours when the countershading would be most effective for concealment, and moving about when it is least effective and in some instances even disadvantage- ous, it is well to look for some other explanation of the phenomenon and not to hastily assume that its purpose is concealment. Thayer's ruptive design theory. is possibly one of the most important ones he has advanced. In certain environments, at any rate, the breaking up of the out- lines of animals in the way suggested is a most effective method of obliteratiou. Nearly all the discussion of. concealing coloration has assumed that all ani- mals have the same powers of vision as man, see things just as man does, and at least one prominent author has expressly declared this to be true. Nothing could be farther from the probability. All men, even, have not equal visual powers. Aside from individual color blindness, there is strong evidence indicating that it is a racial character of some primitive peoples (Science, n. s., xxI, 19o5, p. 680), thus reviving the old theory that in the development of color. perception the colors at the long-wave end of the spectrum were first perceived, and that only a few animals have yet reached the h. igher colors. Birds probably have color vision, though very little is yet known of its extent or universality. Experiments under proper control up to the present time indicate that many of the mammals have not color vision, but only the power of distinguishing between brightness and dullness. Such experiments, to be of any value whatever, are very difficult. There are reasons for the belief that amphibians and reptiles have. only motion vision, which, if true, would nullify the numerous statements about such animals being unable to see their enemies on account of concealing coloration. It is quite certain that all animals are more apt to see any object in motion than a motion~ less one. It is also doubtful whether any animal except the primates has binoc- ular vision, a matter of very great importance. It is generally believed to repre- sent the difference between an ordinary flat photograph and a stereoscopic view, which ]>rings the scene out into sharp .relief. Anyhow, binocular vision is one of the important factors in the perception of solidity, rotundity, etc. Whether the