[Auk 

 April 



The transition between faunas, between provinces, or between 

 adjoining divisions of any grade, is rarely abrupt ; it is impossible 

 to give them absolute boundaries ; yet they may be limited, in a 

 general way, with considerable definiteness. They depend upon 

 climatic conditions, which are in a measure determined or modi- 

 fied by features of topography ; in other words, they are deter- 

 mined by the same conditions that govern the distribution of species. 

 Hence they can seldom be bounded by geographical meridians, 

 or by parallels of latitude, or by political boundaries ; they do, 

 however, closely coincide with certain isothermal lines, which 

 are generally those of the season of reproduction. As tempera- 

 ture is influenced by altitude as well as by latitude, elevated table- 

 lands and mountain ranges deflect the isotherms, in the north- 

 ern hemisphere, far to the southward of their position over the 

 contiguous low country, and furnish congenial homes for northern 

 forms of animals and plants under comparatively low latitudes. 

 Thus in the Appalachian Highlands we find northern types far 

 to the southward of their range in the low country on either side ; 

 and the same is more strikingly apparent in the case of the Rocky 

 Mountains, where northern types extend far down into Mexico, 

 and in the Cascade and Sierra Nevada chain where northern types 

 extend to southern California ; isolated patches of northern life 

 also occur on the summits of detached peaks and outlying ranges 

 throughout the Plateau and Great Basin regions of the West. 

 Northern and southern forms of life thus interdigitate according to 

 elevation of the land. 



While the life of the middle temperate latitudes, at ordinary 

 levels, is so different from that of the high north on the one hand, 

 and from that of the tropics on the other, the change is effected by 

 a very gradual transition between the two extremes. If we sup- 

 pose the life of either of the northern continents to be represented 

 by a cube, and this cube to be composed of two blunt-edged 

 wedges, and the two wedges so placed that one thin edge and one 

 thick edge shall form respectively the base and the top of the 

 cube, the two wedges would represent the fading out of the 

 northern life southward and of the southern life northward ; 

 except that in nature the two elements are diffused through the 

 mass instead of being segregated as in our supposed cube. 



Furthermore, this northern life is largely circumpolar, so that 



