THE ORIGIN OF SPECIES 



selection have, however, fared less enviably when subjected to critical 

 analysis. Two such aspects, adaptive resemblances or coloration, and sex- 

 ual selection, will be discussed below. 



ADAPTIVE RESEMBLANCES 



Perhaps no single factor did more to cause the agnostic reaction than did 

 the excessive speculations of nineteenth century evolutionists on adaptive 

 resemblances. Every possible quirk of nature was interpreted by someone 

 as having adaptive, and therefore selective, value to its possessor. Many 

 speculations were published on the basis of preserved specimens, unsup- 

 ported by field observations. And always the assumption was made that 

 the sensory faculties of the potential predators were similar to those of 

 man. 



The principal types of adaptive coloration are cryptic resemblance, by 

 which the animal blends into its background ( Figure 90 ) ; aposematic, or 

 warning coloration, by which an obnoxious or dangerous animal is made 

 obvious to potential predators; and mimicry, by which one species re- 

 sembles another, presumably taking advantage of aposematism. 



Cryptic Coloration. Cryptic coloration is very general in the animal 

 kingdom. It is useful not only for the protection of animals from their 

 predators, but also to the predators themselves, permitting them to avoid 

 detection by their prey long enough to permit a kill. Perhaps the simplest 

 form is countershading, that is, dark pigmentation on the dorsal surface and 

 light color on the ventral surface. Its protective function may be well illus- 

 trated with a fish. A bird looking down upon the fish will see the dark 

 dorsal surface blending into the darkness of the depths. But a larger fish, 

 looking up from below, sees the light ventral surface blending into the 

 daylight. While the protective effect of such a pattern is plausible, even 

 probable, it is by no means certain that its occurrence depends upon selec- 

 tive value. It has been suggested that the very universality of counter- 

 shading indicates a simple physical cause. Often development of pigment 

 requires exposure to light. As most organisms are exposed to light much 

 more on the dorsal surface than on the ventral, it is to be expected that 

 countershading would be the rule. Some fishes, however, normally swim 

 with the ventral surface up: in these it is the ventral surface which is 

 darkly pigmented and the dorsal surface which is light. Also, this pattern 

 has been experimentally reversed in some fishes by raising them in aquaria 

 lighted from below. But, while such facts demonstrate the means of de- 

 velopment of countershading, they do not disprove its protective value. 

 And that the simlight mechanism is not the whole story is shown by the 

 facts that small surface fishes are typicallv transparent, while abyssal fishes 

 are typically dark-colored. Both would appear to be protective, but both 

 run covmtcr to the light gradient. 



Many cases of cryptic resemblance, however, are less easily disputed. 

 For example, the arctic hare, northern weasels, and ptarmigan molt and 

 grow a coat of white fur or feathers (ptarmigan) in the fall, then revert 

 to a brown coat in the spring. The protective utility is difficult to explain 



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