The geographical distribution of animals, with a study of the relations ...

3chap, i.] INTRODUCTORY. 9Two subjects of a different nature must next engage ourattention. We have to deal with two vast masses of facts,each involving countlessdetails, and requiring subdivision andgrouping to be capable of intelligible treatment. All the continentsand their chief subdivisions, and all the more importantislands of the globe, have to be compared as regards their variousanimal forms. To do this effectively we require a naturaldivision of the earth especially adapted to our purpose ; and weshall have to discuss at some length the reasons for the particularsystem adopted,—a discussion which must to some extentanticipate and summarize the conclusions of the whole work.We have also to deal with many hundreds of families and manythousands of genera ofanimals, and here too a true and naturalclassification is of great importance. We must therefore give aconnected view of the classification adopted in the variousclasses of animals dealt with.And lastly, as the existing distribution of animals is theresult and outcome ofall preceding changes of the earth and ofitsinhabitants, we require as much knowledge as we can get ofthe animals of each country during past geological epochs, inorder to interpret the facts we shall accumulate. We shall,therefore, enter upon a somewhat detailed sketch of the variousforms of extinct animals that have lived upon the earthduringthe Tertiary period ; discuss their migrations at various epochs,the changes of physical geography that they imply, and theextent to which they enable us to determine the birthplaceof 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 ofanimals that would otherwise be insoluble problems ;and, whatis hardly less valuable, will teach us to estimate the comparativeimportance of the various groups of animals, and to avoidthe common error of cutting the gordian knot of each difficultyby vast hypothetical changes in existing continents and oceans—probably the most permanent features of our globe.Vol. I—