CHAP. XVI.] SUMMARY AND CONCLUSION. 159 



America. But the broad conclusions at which we have now 

 arrived seem to rest on a sufficiently extensive basis of facts ; 

 and they lead us to a clearer conception of the mutual relations 

 and comparative importance of the several regions than could 

 be obtained at an earlier stage of our inquiries. 



If our views of the origin of the several regions are correct, 

 it is clear that no mere binary division — into north and south, 

 or into east and west— can be altogether satisfactory, since at 

 the dawn of the Tertiary period we still find our six regions, or 

 what may be termed the rudiments of them, already established. 

 The north and south division truly represents the fact, that the 

 great northern continents are the seat and birth-place of all the 

 higher forms of life, while the southern continents have derived 

 the greater part, if not the whole, of their vertebrate fauna from 

 the north; but it implies the erroneous conclusion, that the 

 chief southern lands — Australia and South America — are more 

 closely related to each other than to the northern continent. 

 The fact, however, is that the fauna of each has been derived, 

 independently, and perhaps at very different times, from the 

 north, with which they therefore have a true genetic relation ; 

 while any intercommunion between themselves has been com- 

 paratively recent and superficial, and has in no way modified 

 the great features of animal life in each. The east and west divi- 

 sion, represents — according to our views — a more fundamental 

 diversity ; since we find the northern continent itself so divided 

 in the earliest Eocene, and even in Cretaceous times ; while we 

 have the strongest proof that South America was peopled from 

 the Nearctic, and Australia and Africa from the Palaearctic 

 region: hence, the Eastern and Western Hemispheres are the two 

 great branches of the tree of life of our globe. But this division, 

 taken by itself, would obscure the facts — firstly, of the close 

 relation and parallelism of the IsTearctic and Palaearctic regions, 

 not only now but as far back as we can clearly trace them in the 

 past ; and, secondly, of the existing radical diversity of the 

 Australian region from the rest of the Eastern Hemisphere. 



Owing to the much greater extent of the old Palsearctic 

 region (including our Oriental), and the greater diversity of 



