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. Hach 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 Palearctic 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 Palearctic 
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 Regions 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 
