392 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



as iinquestionabl}^ the beneficial results of the working of natural 

 selection? Because the dull gray tints of species inhabiting the 

 semiaricl regions of the interior harmonize well with the general 

 gray aspect of their surroundings, is this concordance the result again 

 of the operation of the law of natural selection, the less favorably 

 colored having been weeded out in the struggle for existence? Are 

 the heavy, chdl colors of the humid region of the Northwest the re- 

 sult, again, of the necessary influence of natural selection in perpetu- 

 ating only the individuals whose colors best accord with "their som- 

 ber conditions of environment ? Has the same action brought about 

 the bright, rich coloration of birds, insects, and other animals under 

 the warm, humid conditions of the hotter parts of the earth, preserv- 

 ing the ratio of l^rilliancy of coloration with that of the conditions 

 that everywhere most favor such differentiation? Finally, is the 

 exact correlation of the changes in coloration with the gradual 

 change of climatic conditions in passing from one geographical 

 region to another the result in like manner of the long-continued 

 weeding out of the less favored ? (])r are these modifications sever- 

 ally due to the direct action of the conditions of environment? 



In answering these questions it may be well to glance first at the 

 nature of the theoretical origin of differentiation through the influ- 

 ence of natural selection as expounded by the leading advocates of 

 the theory. As is well known, all the individuals of a species found 

 at the same locality (differences resulting from sex and age aside) 

 are not ail cast in the same mold, but differ constantly, the average 

 range of purely individual variation in general size and in the size 

 of different parts ranging (in birds and mammals) from 8 to 15 or 

 20 per cent of the average size for the species, with a corresponding 

 amomit of variation in color. These variations are found to tend 

 in every conceivable direction, and it of course follows that some of 

 them must be in directions exceptionally favorable to the species. 

 The theory of modification by the action of natural selection only 

 supposes that the stronger or otherwise more favored individuals 

 transmit their favorable qualities to their offspring, and that the 

 latter, in consequence of their inherited advantages, multiply more 

 rapidly than their less favored relatives; that these favorable devia- 

 tions from the parental stock become in subsequent generations more 

 pronounced, and that the original fonn is eventually overpowered 

 and supplanted by its modified descendants. From the same origi- 

 nal stock may be conceived to arise, even simultaneously, other forms 

 diverging in different, though still favorable, directions, these in 

 tui-n giving rise to several lines of descent, occupying perhaps dif- 

 ferent portions of the habitat of the original species,. where they also 

 midtiply and become dominant, and eventually pass on from the 

 stage of incipient species to more or less widely differentiated types. 



