108 DISTRIBUTION OF EXTINCT ANIMALS. [part ii. 



past times ; and when we attempt to generalise the phenomena 

 on a large scale, with the details fresh in our memory, we shall 

 find a reference to the extinct faunas of various epochs to be 

 absolutely necessary. 



The degree of our knowledge of the Palaeontology of various 

 parts of the world is so unequal, that it will not be advisable to 

 treat the subject under each of our six regions. Yet some sub- 

 division must be made, and it seems best to consider separately 

 the extinct animals of the Old and of the ISTew Worlds. Those of 

 Europe and Asia are intimately connected, and throw light on 

 the past changes which have led to the establishment of the 

 three great continental Old "World regions, with their various 

 subdivisions. The wonderful extinct fauna recently discovered 

 in North America, with what was previously known from South 

 temperate America, not only elucidates the past history of the 

 whole continent, but also gives indications of the mutual rela- 

 tions of the eastern and western hemispheres. 



The materials to be dealt with are enormous ; and it will be 

 necessary to confine ourselves to a general summary, with fuller 

 details on those points which directly bear upon our special 

 subject. The objects of most interest to the pure zoologist and 

 to the geologist — those strange forms which are farthest removed 

 from any now living — are of least interest to us, since we aim 

 at tracing the local origin or birthplace of existing genera and 

 families ; and for this purpose animals whose affinities with 

 living forms are altogether doubtful, are of no value whatever. 



The great mass of the vertebrate fossils of the tertiary period 

 consist of mammalia, and this is precisely the class which is of 

 most value in the determination of zoological regions. The 

 animals of the secondary period, though of the highest interest 

 to the zoologist are of little importance to us ; both because of 

 their very uncertain affinities for any existing groups, and also 

 because we can form no adequate notion of the distribution of 

 land and sea in those remote epochs. Our great object is to 

 trace back, step by step, the varying distribution of the chief 

 forms of life ; and to deduce, wherever possible, the physical 

 changes which must have accompanied or caused such changes. 



