236 STUDIES, SCIENTIFIC AND SOCIAL chap. 



separate regions. It is this one question only that 

 I propose to discuss in the present chapter; whether 

 the Palsearctic and Nearctic Regions, as defined by 

 Sclater, present so many resemblances and so few differ- 

 ences that they can be reasonably considered to form 

 one region as homogeneous as are most of the other 

 regions. 



The reason why so much difference of opinion exists on 

 this point, when the very same facts are before all the 

 enquirers, seems to be that they treat the facts in different 

 ways. In the first place, it appears to me that far too 

 much stress is laid upon the comparatively small number 

 of absolutely peculiar genera or families in the two tem- 

 perate regions. That may, or may not, be a reasonable 

 ground for deciding that the regions in question are not 

 of equal rank with some other regions; but it cannot 

 justify the union of the two areas unless it can be 

 shown that they are to a large extent homogeneous — 

 that, in fact, the resemblances are more numerous and 

 more important than the differences. In the areas we 

 are discussing, however, this is decidedly not the case, 

 since the facts clearly show that the differences very far 

 surpass the resemblances — that the differences are, in 

 fact, fundamental, and are far greater than can be found 

 in the separate halves of any of the other regions, unless 

 they are so divided as to be very unequal in area or to 

 present very great diversity of climate. But the Palsearctic 

 and Nearctic Regions are, roughly speaking, equal in area, 

 while they both exhibit the very same range of climate. 

 Any difference that exists between them must, therefore, 

 be due to more fundamental causes ; and the most funda- 

 mental cause is, that in each case the existing fauna 

 is descended from an ancient, equally distinct, fauna, 

 modified in different ways by immigration from adjacent 

 areas. 



In dealing with any question of this kind, it is very 

 important that the genera and family groups adopted shall 

 be the same as far as possible throughout the whole area. 

 In order to ensure this, I have, for mammalia, adopted 

 those of Flower and Lydekker in their Mammah Living 



