350 
DARWINISM 
CHAP. 
surprised that many widespread forms in either continent 
have not crossed into the other; and that while the skunks 
(Mephitis), the pouched rats (Saccomyidse), and the turkeys 
(Meleagris) are confined to America, the pigs and the hedge¬ 
hogs, the true flycatchers and the pheasants are found only 
in the Euro-Asiatic continent. But, just as there have been 
periods which facilitated intermigration between America and 
the Old World, there have almost certainly been periods, 
perhaps of long duration even geologically, when these con¬ 
tinents have been separated by seas as wide as, or even wider 
than, those of the present day; and thus may be explained 
such curious anomalies as the origination of the camel-tribe in 
America, and its entrance into Asia in comparatively recent 
Tertiary times, while the introduction of oxen and bears into 
America from the Euro-Asiatic continent appears to have been 
equally recent. 1 
We shall find on examination that this view of the general 
permanence of the oceanic and continental areas, with constant 
minor fluctuations of land and sea over the whole extent of 
the latter, enables us to understand, and offer a rational 
explanation of, most of the difficult problems of geographical 
distribution ; and further, that our power of doing this is in 
direct proportion to our acquaintance with the distribution of 
fossil forms of life during the Tertiary period. We must, also, 
take due note of many other facts of almost equal importance 
for a due appreciation of the problems presented for solution, 
the most essential being, the various powers of dispersal 
possessed by the different groups of animals and plants, the 
geological antiquity of the species and genera, and the width 
and depth of the seas which separate the countries they 
inhabit. A few illustrations will now be given of the way in 
which these branches of knowledge enable us to deal Avith the 
difficulties and anomalies that present themselves. 
The Distribution of Marsupials. 
This singular and loAvly organised type of mammals con¬ 
stitutes almost the sole representative of the class in Australia 
1 For some details of these migrations, see the author’s Geographical 
Distribution of Animals, vol. i. p. 140 ; also Heilprin’s Geographical and 
Geological Distribution of Animals. 
