OF LIFE IN GENERAL. 327 



especially in its length." This increase is quite marked 

 in the genera Quiscalus, Agel&us, Geothlypis, Troglo- 

 dytes, Seiurus, etc. 



As to the causes of these geographical variations, it 

 is of course impossible to ascribe them with any cer- 

 tainty even to the indirect effects of change of environ- 

 ment, much less to the direct. Still, as Allen points 

 out, there is often a somewhat close correlation be- 

 tween geographical varieties and the meteorological 

 peculiarities of the regions in which they occur, which 

 suggests a connection of some sort between the two. 

 The increase in colour towards the south coincides 

 with the increase in the intensity of the sun's rays, and 

 in the humidity of the climate. The increase in colour 

 observed in birds on passing from East to West 

 seems also to coincide with an increase of humidity, 

 " the darker representatives of any species occurring 

 where the annual rainfall is greatest, and the palest 

 where it is least." This coincidence occurs not only in 

 the birds of the United States, to such a degree that 

 Allen says he knows of no exception, but in Europe 

 also. Thus birds from the Scandinavian coast are 

 very much darker than in central Europe, where the 

 rainfall is only half as great. Allen says that this cor- 

 relation of brighter and deeper tint with increased 

 humidity is exhibited by the mammals of these dis- 

 tricts, as well as by the birds. 



The differences in the local races of certain Mam- 

 mals are even more striking than in those of the birds. 

 The Canidse, for instance, are represented in North 

 America by six species, viz., gray wolf, common fox, 

 gray fox, coyote, arctic fox, and kit fox, of which the 



