MUSEUM OF COMPARATIVE ZOOLOGY. 383 



ate zone into five subzones, — a Temperate proper,, a Subtemperate, a 

 Warm Temperate, a Cold Temperate, and a Subfrigid. These zones 

 are equally recognizable in the distribution of terrestrial life ; but, owing 

 to inequalities of its surface, they are of course less regular on the 

 land than on the oceans. 



The zones and subzones, or the Provinces and the minor phytological 

 and zoological divisions of the globe, are usually not trenchantly de- 

 fined. Their boundaries being determined by climatic conditions, the 

 transition between adjacent zones, or between ontological divisions of 

 •whatever rank, is rarely abrupt ; like the climatic zones, they blend 

 more or less at their edges, their boundaries being strongly marked only 

 in regions possessing a highly varied surface, as in mountainous dis- 

 tricts. They are, nevertheless, easily recognizable, and can be ap- 

 proximately defined. Generally the dividing lines are more or less 

 undulating, and, being determined indirectly by chains of mountains 

 and other physical barriers, adjoining faunae and florae, and even adjoin- 

 ing provinces and realms, almost always interdigitate, and frequently 

 enclose isolated areas of others, as will be presently shown in describ- 

 ing the ornithological fauna? of Eastern North America. 



The boundaries of faunae and florae, like the range of species, are 

 determined indirectly by elevations and depressions of the earth's sur- 

 face, these variations in the altitude of the land producing varying 

 conditions of temperature and humidity, which latter, as already stated, 

 are the direct limiting influences of species, and of tn*e botanical and 

 zoological divisions of the globe. The permanency of their boundaries 

 hence depends upon the constancy of the physiographic conditions of 

 these areas, a migration of species, and of faunae and florae, necessarily 

 following changes in these conditions. That such migrations have 

 taken place is evident from the occurrence in the post-tertiary 

 deposits of the warm temperate latitudes of the fossil remains of 

 species found now only in the cold temperate and arctic regions, and 

 in the tertiary strata of high latitudes of the remains of other species 

 whose nearest allies are now found in the warm tempeiatu and sub- 

 tropical zones. These facts indicate clearly the great changes in 

 temperature that have repeatedly occurred at given localities during 

 the earth's history. In respect to existing animals, however, it is dif- 

 ficult to determine how much their known recession northward, as of 

 the reindeer, for example, is due to climatic changes, and how much to 



