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 Avhich 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 Avide 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, Avhile the introduction of oxen and bears into 

 America from the Euro- Asiatic continent ajjpears to have been 

 equally recent. ^ 



We shall find on examination that this vieAv 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 

 exjDlanation of, most of the difflcult 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 gi^oups 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 Avay in 

 which these branches of knowledge enable us to deal with the 

 difficulties and anomalies that present themselves. 



The Distribution of Marsupials. 



This singular and lowly 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 Geogra;phical 

 Distribution of Animals, vol. i. p. 140 ; also Heilprin's Geographical and 

 Geological Distribution of Animals. 



