MUSEUM OF COMPARATIVE ZOOLOGY. 381 



provinces. Its southern boundary may be considered as the northern 

 limit of forest-trees, or about the isotheral of 50° F. 



The North Temperate Realm presents a more varied character, and 

 is divisible into an American Region and an Eur opceo- Asiatic Region, 

 each of which is divisible into provinces, districts, and faunae and florae. 

 Its boundaries may be provisionally considered as the isotherms of 32° 

 and 70° F. 



The American Tropical Realm, and also the Indo- African Tropical 

 Realm, may be regarded as bounded by the isotherms of 70° F. The 

 first is far more homogeneous than the second. Though the American 

 Tropical Realm is perhaps not divisible into distinct regions, it cer- 

 tainly embraces several provinces and districts, and is rich in faunae 

 and florae. The Indo-African Tropical Realm may be divided into an 

 African Region and an Indian Region, each composed of several prov- 

 inces and districts, and a great number of faunae and florae. 



The South American Temperate Realm embraces that part of South 

 America south of the isotherm of 70° F. ; the African Temperate 

 Realm includes that part of Africa south of the same isotherm, whilst 

 the Antarctic Realm is restricted to the antarctic islands. 



The Australian Realm, embracing Australia, New Zealand, New 

 Guinea, and their dependent islands, including those to the eastward 

 as far as Timor and Celebes, is zoologically as distinct from the other 

 primary regions as it is in its geographical position. It is divisible 

 into a Temperate and a Tropical Region, the former embracing New 

 Zealand and the southern third of Australia. Each of these regions 

 includes two or three well-marked provinces. 



The above division of the earth's surface* avoids the arbitrary 

 partitioning of an almost homogeneous Arctic Realm between two 



* It is not within the scope of the present article to trace the subdivisions of the 

 earth's surface in relation to the distribution of its organic life any further than to 

 furnish illustrations of the general principles according to which it is believed animals 

 and plants are distributed, and by which the land surface of the earth is divided. De- 

 termining the rank of the several divisions by the amount of variation from others they 

 present, it is found, as indicated above, that the divisions of co-ordinate rank increase 

 in number to the southward. The Arctic Realm is homogeneous to such an extent as 

 not to admit of divisions of a higher grade than faunae and flora;. In the Temperate 

 Realm the animals and plants of the Eastern and Western hemispheres are, as a whole, 

 so far different as to admit of the division of this zone into two grand divisions (divis- 

 ions of the second order), with other divisions between these and the ultimate ones. 

 In the Tropical Realm the differences in the life of the two hemispheres is so great as 



