(i-l Merriam Geographic Distribution of Life. 



menting or being' the complement of the other for water being 

 the medium in which the species live, the bodies of water with 

 their prolongations and extensions, as hays, rivers, and lakes, 

 must be studied as entities, just as we study a continent with its 

 peninsulas and outlying islands the means of access to a given 

 body of water being the principal factor in determining the 

 water-area to which its aquatic life belongs. And it should be 

 remarked that aquatic mammals, as seals and cetaceans, and 

 aquatic birds, as ducks and gulls, conform in the main to the laws 

 and areas of aquatic distribution and should not be taken into 

 account in studying the distribution of terrestrial forms of life. 



Gill has said with much truth : " There appears to be a total 

 want of correlation between the inland and marine faunas, and 

 a positive incongruity, and even contrast, between the two." 

 (Proc. Biol. Soc. Wash.. II, 1884, 32.) 



PRINCIPLES ON WHICH BIO-GEOGRAPHIC REGIONS SHOULD BE 

 ESTABLISHED. 



Wallace, in writing of the principles on which Zoological 

 regions should be formed, expresses the opinion that " conveni- 

 ence, intelligibility, and custom, should largely guide us." But 

 I quite agree with America's most distinguished and philosophic 

 writer on distribution, Dr. J. A. Allen, that in marking off the 

 life regions and subregions of the earth, truth should not be 

 sacrificed to convenience; and I see no reason why a homo- 

 geneous circumpolar fauna of great geographic extent should be 

 split up into primary regions possessing comparatively few 

 peculiar types simply because a water separation happens to 

 exist in the present geologic period ; nor is it evident why one 

 of the resulting feeble divisions should be granted higher rank 

 than a region of much less geographic extent comprising several 

 times as many peculiar types. Hence the divisions here recog- 

 nized, and the rank assigned them, are based as far as possible 

 upon the relative numbers of distinctive types of mammals, 

 birds, reptiles, and plants they contain, with due reference to the 

 steady multiplication of species, genera, and higher groups from 

 the poles toward the tropics. Mammals have been chiefly used 

 as illustrations because they answer the purpose better than a in- 

 other single group, and because it is clearly impossible in a brief 

 essav of this character to enumerate such a multitude of forms 

 as would be necessarv were equal consideration accorded to each 

 class. 



