CHAP. I.] INTRODUCTORY. 9 
Two subjects of a different nature must next engage our 
attention. We have to deal with two vast masses of facts, 
each involving countless details, and requiring subdivision and 
grouping to be capable of intelligible treatment. All the con- 
tinents and their chief subdivisions, and all the more important 
islands of the globe, have to be compared as regards their vari- 
ous animal forms. To do this effectively we require a natural 
division of the earth especially adapted to our purpose; and we 
shall have to discuss at some length the reasons for the particular 
system adopted,—a discussion which must to some extent 
anticipate and summarize the conclusions of the whole work. 
We have also to deal with many hundreds of families and many 
thousands of genera of animals, and here too a true and natural 
classification is of great importance. We must therefore give a 
connected view of the classification adopted in the various 
classes of animals dealt with. 
And lastly, as the existing distribution of animals is the 
result and outcome of all preceding changes of the earth and of 
its inhabitants, we require as much knowledge as we can get of 
the animals of each country during past geological epochs, in 
order to interpret the facts we shall accumulate. We shall, 
therefore, enter upon a somewhat detailed sketch of the various 
forms of extinct animals that have lived upon the earth during 
the Tertiary period; discuss their migrations at various epochs, 
the changes of physical geography that they imply, and the 
extent to which they enable us to determine the birthplace 
of certain families and genera. 
The preliminary studies above enumerated will, it is believed, 
enable us to see the bearing of many facts in the distribution of 
animals that would otherwise be insoluble problems; and, what 
is hardly less valuable, will teach us to estimate the compara- 
tive importance of the various groups of animals, and to avoid 
the common error of cutting the gordian knot of each difficulty 
by vast hypothetical changes in existing continents and oceans 
—probably the most permanent features of our globe. 
Vout. L—3 ; 
