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 Paleontology 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 New 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 
inost 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. 
