398 PHYSICAL CONDITION'S IN GENESIS OP SPECIES. 



of which they mnhiply till they outnumber, crowd out, and finally 

 destroy the less-favored form from which they originated. 



It is hardly conceivable, for example, how the peculiar structure 

 Been in the woodpecker, the kingfisher, the swift, the heron, or the 

 duck, or the peculiar dentition and correlated characters of the 

 rodents, the ruminants, or the shrews and moles, as compared with 

 the Carnivora, can have been initiated by the direct action of climatic 

 conditions, however much other conditions of environment may have 

 favored the development of these diverse types. 



Having thus far mainly detailed merely facts and coincidences 

 relating to the subject of variation with locality, it may be well in 

 conclusion to consider more fully some of the possible or probable 

 causes of purely geographical variation. In regard to geographical 

 variation in color, it seems evident that high temperature, conjoined 

 with moisture, favors increase of color, and especially the acquisition 

 of lustrous tints, while moisture alone favors simply increase in depth 

 or the production of dull, heavy, and especially fuscous phases of 

 coloration ; on the other hand, that aridity and exposure favor the 

 loss of color. The latter is due apparently no less to the influence of a 

 dry and often intensely heated atmosphere than to the direct action 

 of light intensified by the reflection of the sun''s rays from almost 

 verdureless sands. That the latter conditions act powerfully in 

 blanching color there is most abundant proof. Hence we have the 

 necessary correlation of increase of bright rich tints of coloration 

 with the increase of atmospheric humidity. In respect to the enlarge- 

 ment of peripheral parts at the southward, it is obvious tJiat a high 

 temperature favors the more rapid circulation of the blood in these 

 parts, while, as is well known, a low temperature produces the oppo- 

 site effect and necessarily retards their development. 



With the decrease in size among birds, there has been observed a 

 decrease of Advacity and deterioration of song, which may reasonably 

 be attributed to the enervating influence of a high temperature. 

 Since the northern types of animals reach their highest physical 

 development toward the northward, it seems fair to suppose that 

 decrease in size southward may be directly due to the enfeebling 

 influences of increase of temperature, since certainly it can not be 

 attributed, in the majority of cases at least, to greater scarcity of 

 food, for in many instances just the reverse obtains. This supposi- 

 tion is in accordance with the known effects of similar climatic con- 

 ditions upon the northern races of man, which reach their greatest 

 vigor and highest intellectual status under temperate conditions of 

 climate, and deteriorate, both physically and mentally, on removing 

 to intertropical regions. Again, the mammals and birds of the 

 United States reach their maximum size within the United States 



