CHAT*. 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 Hapalidse as compared with the Old World Apes and Mon- 
keys ; while its Cracidse 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- 
thionidee and Megapodiidse ; the Lemurs and Insectivora pre- 
served in Madagascar are represented by the Mascarene Didkke; 
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 Coliidee and Musophagidm 
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 
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