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

108 DISTRIBUTION OF EXTINCT ANIMALS. [part ii.past times ; and when we attempt to generalise the phenomenaon a large scale, with the details fresh in our memory, we shallfind a reference to the extinct faunas of various epochs to beabsolutely necessary.The decree of our knowledge of the Palaeontology of variousparts of the world is so unequal, that it will not be advisable totreat the subject under each of our six regions.Yet some subdivisionmust be made, and it seems best to consider separatelythe extinct animals of the Old and of the New Worlds. Those ofEurope and Asia are intimately connected, and throw light onthe past changes which have led to the establishment of thethree great continental Old World regions, with their varioussubdivisions. The wonderful extinct fauna recently discoveredin North America, with what was previously known from Southtemperate America, not only elucidates the past history of thewhole continent, but also gives indications of the mutual relationsof the eastern and western hemispheres.The materials to be dealt with are enormous ; and it will benecessary to confine ourselves to a general summary, with fullerdetails on those points which directly bear upon our specialsubject. The objects of most interest to the pure zoologist andto the geologist—those strange forms which are farthest removedfrom any now living—are of least interest to us, since we aimat tracing the local origin or birthplace of existing genera andfamilies ; and for this purpose animals whose affinities withliving forms are altogether doubtful, are of no value whatever.The great mass of the vertebrate fossils of the tertiary periodconsist of mammalia, and this is precisely the class which is ofmost value in the determination of zoological regions. Theanimals of the secondary period, though of thehighest interestto the zoologist are of little importance to us ; both because oftheir very uncertain affinities for any existing groups, and alsobecause we can form no adequate notion of the distribution ofland and sea in those remote epochs. Our great object is totrace back, step by step, the varying distribution of the chiefforms of life ; and to deduce, wherever possible, the physicalchanges which must have accompanied or caused such changes.