COLORATION IN POLISTES. 75 



CONSIDERATIONS WITH RESPECT TO VARIOUS THEORIES OF EVOLUTION 



After the discussion in the preceding section little need be said of 

 the bearing of the facts on theory. While the evidence from the 

 study of individual variation is decidedly in favor of the occurrence 

 of numerous almost imperceptible differences and against discontinuous 

 variation, I have been able to find in the environmental condition no 

 adequate factor for the selection of such variations which might lead 

 to the evolution of divergent species. The observations on the devel- 

 opment of the color pattern in the various species are in accord with 

 Elmer's theory of orthogenesis — that is, the color pattern for both the 

 individual and the species may be regarded as arising from the tendency 

 of the organism to pause at some particular stage of the process of 

 pigmentation— but this tendency is due not to any inherent property of 

 the organism, but rather to the differing environal conditions to which 

 it is subjected. These conditions affect the metabolism of the insect in 

 definite directions, and through it the products of metabolism, in this 

 instance the kind and amount of pigment. 



Specific differentiation springs from individual variation, and both 

 are in large measure due to external conditions. Such characters 

 appear to belong to the class of auto-adaptive characters. Study of 

 the specific distinction of many other orders prove such auto-adaptive 

 characters to be of wide occurrence in the animal world. 



The following passage is quoted from J. A. Allen's (i) Influence 

 of Physical Conditions in the Genesis of Species : 



We find among the mammals and birds of the United States three strongly 

 marked phases of color-differentiation among representatives of the same species, 

 characterizing the three most strongly marked climatal regions— a bright, strongly 

 colored form east of the Great Plains, a pallid form over the dry central region, 

 and a deeply colored fuscous form over the rainy, heavily wooded region of the 

 northwest coast. Examples of this differentiation are afforded by apparently all 

 the species whose habitats extend entirely across the continent, the several local 

 forms being in some species only more strongly marked than in others. Among 

 mammals illustrations are afforded by different species of squirrels, hares, mice, 

 lynxes, deer, etc. ; and among birds by six or eight species of sparrows, a number 

 of woodpeckers, several fly-catchers, thrushes, and warblers, the meadow lark, 

 various hawks, owls, etc. Generally these several geographical forms were orig- 

 inally described as distinct species, and many of them are still thought worthy of 

 recognition by varietal names As intermediate links began to be discovered, they 

 were at first looked upon as the result of hybridity between the supposed distinct 

 species whose characters they respectively combined, but eventually such links 

 were found to be too frequent and too general over the areas where the habitats 

 of the several forms come together to render such a supposition longer tenable, 

 it finally appearing evident that they were only the connecting forms between 

 merely local races or incipient species. 



