PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 391 



The local races of any given region, as compared collectively with 

 those of contiguous regions and the manner of their mutual intergra- 

 dation, point plainly to some general or widely acting cause of differ- 

 entiation. This is indicated by the constancy of the results, so many 

 species, belonging to numerous and widely distinct groups, being simi- 

 larly affected. Will the fortuitous, spontaneous results of natural 

 selection yield a satisfactory explanation of these phenomena, or must 

 we seek some more uniform and definitely acting cause? To briefly 

 summarize the results above detailed, we have a somewhat uniform 

 increase of size in some given direction, affecting- many species 

 simultaneously and similarly over the same areas. We have a fre- 

 quent enlargement of peripheral parts, affecting not a few, but many, 

 species, and all in a similar manner, though in varying degrees. We 

 have a very general increase in the depth or intensity of colors south- 

 w\ard, a general loss of color in approaching the central arid portions 

 of the continent, and again an excessive increment of color under 

 still different climatic conditions and over a different area. We find 

 the increase of size among the individuals of any given species to be 

 quite uniformly in the direction of the center of distribution of the 

 group to which the species belongs, this being especially well nuirked 

 in mammals. We find the increase in the size of peripheral parts, as 

 the external ear and the length of the pelage in mammals and the 

 size of the bill and length of the tail in birds, to be in the direction 

 of the regions Avhere these parts meet, respectively, their greatest 

 development— the increase m color (especially among birds) toward 

 the region where are developed the richest and most lustrous tints; 

 the loss of color in the direction of the region where the greatest 

 general pallor prevails. 



We find again that the enlargement of peripheral parts correlates 

 with increase of temperature; the southward increase of color with 

 an increase of atmospheric humidity and temperature, and conse- 

 quently with the protective influences of luxuriant arboreal vegeta- 

 tion and clouds; and, conversely, the loss of color accompanying 

 excessive aridity, a scanty vegetation, and an almost cloudless sky, 

 the conditions, in short, of all others the most powerfully effective in 

 the blanching of color; and again, the somber, dusky, tints of the 

 northwest coast accompanying the most humid conditions of climate 

 and the conditions generally most favorable for the protection or 

 preservation of color. Are these merely accidental coincidences, or 

 are they the evident results of cause and effect? Because the white 

 winter livery of some of the northern species is more protective 

 against cold than darker tints would he, or aids in concealing them in 

 some cases (as in the hares and ptarmigans) from their enemies, or 

 in other cases (as in the ermines and the arctic fox) tends to aid 

 them in stealing unperceived upon their prey, are they to be regarded 



