The geographical distribution of animals, with a study of the relations ...

;chap, ix.] ORDER OF SUCCESSION OF THE REGIONS. 175regions ; and a careful inspection of the diagrams themselves,taken in their entirety, will, it is believed, show that this isthe most natural plan, and most truly exhibits the relations ofthe several regions.In the portion of our work now commencing, we are not,however, by any means bound to begin at either end of thisseries. Each region is studied by itself, but reference will oftenhave to be made to all the other regions ;and wherever webegin, we must occasionally refer to facts which will be givenfurther on. As, however, the great northern continents formthe central mass from which the southern regions, as it were,diverge, and as the Pala?arctic region isboth more extensive andmuch better known than any other, it undoubtedly forms themost convenient starting-point for our proposed survey ofthe zoological history of the earth. We thus pass from thebetter known to the less known—from Europe to Africa andtropical Asia, and thence to Australia, completing the series ofregions of the Eastern Hemisphere.Beginning again with theNeotropical region, we pass to the Nearctic, which has suchstriking relations with the preceding and with the Palsearcticregion, that it can only be properly understood by constantreference to both. We thus keep separate the Eastern and Westernhemispheres, which form, from our point of view, themost radical and most suggestivedivision of terrestrial faunasand as we are able to make this also the dividing point of ourtwo volumes, reference to the work will be thereby facilitated.Cosmopolitan Groups.— Before proceeding to sketch the zoologicalfeatures of the several Begions it will be well to noticethose family groups which belong to the earth as a whole, andwhich are so widely and universally distributed over itthat itwill be unnecessary, in some cases, to do more than refer tothem under the separate geographical divisions.The only absolutely cosmopolitan families of Mammalia arethose which are aerial or marine; and this is one of the strikingproofs that their distribution has been effected by natural causes,and that the permanence of barriers is one of the chief