chap, ix.] ORDER OF SUCCESSION OF THE REGIONS. 175 



regions ; and a careful inspection of the diagrams themselves, 

 taken in their entirety, will, it is believed, show that this is 

 the most natural plan, and most truly exhibits the relations of 

 the several regions. 



In the portion of our work now commencing, we are not, 

 however, by any means bound to begin at either end of this 

 series. Each region is studied by itself, but reference will often 

 have to be made to all the other regions ; and wherever we 

 begin, we must occasionally refer to facts which will be given 

 further on. As, however, the great northern continents form 

 the central mass from which the southern regions, as it were, 

 diverge, and as the Palsearctic region is both more extensive and 

 much better known than any other, it undoubtedly forms the 

 most convenient starting-point for our proposed survey of 

 the zoological history of the earth. We thus pass from the 

 better known to the less known — from Europe to Africa and 

 tropical Asia, and thence to Australia, completing the series of 

 regions of the Eastern Hemisphere. Beginning again with the 

 Neotropical region, we pass to the Nearctic, which has such 

 striking relations with the preceding and with the Paleearctic 

 region, that it can only be properly understood by constant 

 reference to both. We thus keep separate the Eastern and West- 

 ern hemispheres, which form, from our point of view, the 

 most radical and most suggestive division of terrestrial faunas ; 

 and as we are able to make this also the dividing point of our 

 two volumes, reference to the work will be thereby facilitated. 



Cosmopolitan Groups. — Before proceeding to sketch the zoo- 

 logical features of the several Eegions it will be well to notice 

 those family groups which belong to the earth as a whole, and 

 which are so widely and universally distributed over it that it 

 will be unnecessary, in some cases, to do more than refer to 

 them under the separate geographical divisions. 



The only absolutely cosmopolitan families of Mammalia are 

 those which are aerial or marine; and this is one of the striking 

 proofs that their distribution has been effected by natural causes, 

 and that the permanence of barriers is one of the chief 



