10 GEOGEAPHICAL DISTRIBUTION. 



marsupials, and they have likewise been considered to afford proof 

 of a land connection such as has been indicated. A serious diffi- 

 culty, however, that lies in the way of this explanation is the 

 important fact that none of the characteristic African or South 

 American mammals are found in Australia, for it might justly be 

 contended that if a migration or transferrence was effected in one 

 direction, it could have been effected in the opposite direction as 

 well. But that such reciprocal distribution did not obtain is very 

 nearly certain. It may, indeed, be assumed that at the time of a 

 possible Australian migration the extremities of the southern con- 

 tinents were not yet inhabited ; but this is very unlikely. Or, it 

 may be further assumed, with Riitimeyer, that the animals under 

 consideration had a polar origin, and that they were distributed 

 northward along continental lines that possibly now lie buried 

 beneath the sea ; but positive evidence in this direction is still 

 wholly wanting. An element in the problem which very materi- 

 ally narrows the issue is the circumstance that marsupial remains 

 have been found in the temperate regions of the Northern Hemi- 

 sphere, and in both Europe and North America in deposits as an- 

 cient as the Triassic period. In this upper tract, therefore, we 

 find a possible and more probable clue towards the explanation of 

 the existing distribution of the animals in question ; and if it be 

 objected that some such living forms ought still to be found in 

 the connecting region, the fact, nevertheless, remains that they did 

 there once exist, but have since become largely extinct. 



It will be evident that the key to the solution of the more 

 marked peculiarities of modern distribution must be sought in the 

 records of the past, for in the comparison between existing and 

 preexisting faunas alone can we expect to determine the condi- 

 tions upon which present faunas were established, and to ascertain 

 the dates of their respective appearances or antiquity. In most 

 regions of the earth's surface a most intimate relationship links 

 together the existing fauna and the fauna of the geological period 

 or periods immediately preceding. The Pliocene and Post-Pliocene 

 marine shell-fish faunas of the Western United States are practically 

 identical with the equivalent fauna of the (modern) adjoining seas ; 

 the Post-Pliocene mammals of Britain are such as still roam about 

 the land, although they include numerous forms which no longer 

 exist there; in India a large proportion of the mammalian types 



