CHAP, xxiii.] SUMMARY AND CONCLUSION. 545 



the Temperate and Tropical zones, and to render further intermi- 

 gration almost impossible. 



In a few cases — of which the Eodents in Australia and the 

 pigs in Austro-Malaya are perhaps the most striking examples 

 — the distribution of land-mammals has been effected by a sea- 

 passage either by swimming or on floating vegetation ; but, as 

 a rule, we may be sure that the migrations of mammalia have 

 taken place over the land; and their presence on islands is, 

 therefore, a clear indication that these have been once connected 

 with a continent. The present class of animals thus affords the 

 best evidence of the past history of the land surface of our 

 globe ; and we have chiefly relied upon it in sketching out (in 

 Part III.) the probable changes which each of our great regions 

 has undergone. 



Birds. 



Although birds are, of all land-vertebrates, the best able to 

 cross seas and oceans, it is remarkable how closely the main 

 features of their distribution correspond with those of the Mam- 

 malia. South America possesses the low Formicaroid type of 

 Passeres, — which, compared with the more highly developed 

 forms of the Eastern Hemisphere, is analogous to the Cebidse 

 and Hapalidae as compared with the Old World Apes and Mon- 

 keys ; while its Cracidae as compared with the Pheasants and 

 Grouse, may be considered parallel to the Edentata as compared 

 with the Ungulates of the Old World. The Marsupials of 

 America and Australia, are paralleled, among birds, in the Stru- 

 thionidse and Megapodiidse ; the Lemurs and Insectivora pre- 

 served in Madagascar are represented by the Mascarene Dididse ; 

 the absence of Deer and Bears from Africa is analogous to the 

 absence of Wrens, Creepers, and Pheasants ; while the African 

 Hyracidse and Chrysochloridae among mammals, may well be 

 compared with the equally peculiar Coliidse and Musophagidae 

 among birds. 



From these and many other similarities of distribution, it is 

 clear that birds have, as a rule, followed the same great lines of 

 migration as mammalia ; and that oceans, seas, and deserts, have 



