CHAP. 1.] INTKODUCTUKY. 



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 efffctively 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 epoclis, 

 the changes of pliysical 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. 



