434 NATURAL SCIENCE. June. 



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 former 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 differences in climate. But the 

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

 while they both exhibit the very same range of climate. Any dif- 

 ference that exists between them must, therefore, be due to more funda- 

 mental causes ; and the most fundamental 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 

 "Mammals Living and Extinct." In the case of the birds, I am 

 indebted to my friend Mr. H. E. Dresser, both for completing his 

 " List of European Birds " so as to include those of the whole 

 Palaearctic region, and also for so harmonising his genera with those 

 of the "A. O. U. Check List of North American Birds," as to 

 combine the whole into a single series. As Mr. Dresser has made a 

 study of North American birds as well as of those of the Palaearctic 

 Region with which his name is more especially associated, the result 

 is probably as fair and unbiased, for the comparison in question, as 

 can be arrived at. In the case of both regions, I have rejected all 

 those genera in which a single species only just enters the region 

 from the adjacent region to which it properly belongs. Here, again, 

 there is, of course, room for difference of opinion ; but the doubtful 

 cases are not sufficiently numerous to introduce any important 

 uncertainty in the result. 



For both mammals and birds I adopt the same system of 

 presenting the facts which, after much consideration and many 

 trials, seems to me to be the best. The families and genera are 

 arranged in three columns — the first giving those which are Palae- 

 arctic but not Nearctic ; the third those that are Nearctic but not 

 Palaearctic ; the middle column giving those which are common to 

 the two regions — so that by combining this with either of the others 

 we obtain the entire fauna of that region. The results are then 

 summarised in other tables and briefly discussed. We begin with 

 the mammals : — 



