PDF - Wallace Online

CHAP, xxui.] SUMMARY AND CONCLUSION. 545
the Temperate and Tropical zones, and to render further intermi-
gration ahnost impossible.
In a few cases— of which the Rodents 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 aU 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 Cebidffi
and Hapalidte as compared with the Old World Apes and Monkeys
; 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-
thionidae 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
Hyracidte and Chrysochloridse among mammals, may well be
compared with the eq^ually peculiar Coliidae and Musophagidse
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
;