MUSEUM OF COMPARATIVE ZOOLOGY. 379 



nized. The boundaries of realms and provinces have often been arbi- 

 trarily fixed, inasmuch as they have been frequently limited and named 

 in conformity to the continental areas, regardless of the fundamental 

 law of the distribution of life in circumpolar zones.* 



In addition to the law of the circumpolar distribution of life in zones, 

 another may be recognized, namely, that of a differentiation from the 

 north southward, since in passing from the northern pole to the. equator 

 we meet with a constant and accelerated divergence in the character 

 of the animals and plants of successive regions of the continent. More 

 or less related to the last is a third law of differentiation, namely, a 

 divergence of the life of given portions of continental areas from that 

 of the corresponding portions of other continents, in proportion to the 

 oceanic space separating such corresponding regions. As evidence of 

 this fact we have but to compare successively the life of the north 

 temperate, tropical, and south temperate zones of the Western hemi- 

 sphere with the life of the corresponding zones of the Eastern hemi- 

 sphere ; or that of Australia with the life of the other continents, as a 

 whole ; or that of tropical Asia with Africa or South America. A 

 comparison of Africa with South America, and the fauna? and florae 

 of islands with those of the different continents, further corroborates 

 this law. There is, furthermore, a correlation between the diversity 



term. Realm, region, fauna and flora, and province, have been also successively used in 

 the same sense, and also for divisions of subordinate rank, and in different ways by even 

 the same writers. In regard to the names of the divisions of the second, third, and 

 fourth rank, there is an equal want of uniformity in the use of the terms by which they 

 have been designated. As being most convenient and least opposed to current usage, 

 the following schedule of -names for *'ie primary and subordinate divisions has been 

 °dopted in the present paper: — 



Realms for divisions of tlie first rank. 



Regions for divisions of the second rank. 



Provinces for divisions of the third rank. 



Districts for divisions of the fourth rank. 



Fauna and flora for the smallest or ultimate divisions, like the bird faunae of Eastern 

 North America, presently to be characterized. 



Intermediate divisions to some of those above mentioned may in special cases be re- 

 quired; but until the necessity for them is made apparent, no names for such need be 

 proposed. 



* Sclater, P. L. " On the general Geographical Distribution of the Members of the 

 Class Aves," Jour, of the Proc. of the Linnsean Society, Vol. II, Zoology, pp. 130- 149, 

 1858. The divisions proposed by this author have been quite generally adopted, but 

 without corroboration, or apparently a critical examination of their merits 



