INTRODUCTION. 



13 



may so express myself, the raw material which all 

 the various influences that can bring about more or 

 less well-defined variations, have worked up within 

 the limits assigned by the distribution of land and 

 water. 



The present geographical distributionof mammals 

 is thus intimately related to their origin. Land 

 mammals descended from ancestors which were 

 restricted to a continent, forming an island sur- 

 rounded by the waters of the ocean, have not been 

 able to develop on another continent inaccessible 

 to them, however favourable the conditions of life 

 there might be. Broad rivers, high mountain chains, 

 deserts, and marshes could not but hinder the 

 advance of certain types, and have actually pre- 

 vented their introduction into regions which were 

 cut off by barriers of that nature. 



Every species, however strong or weak its powers 

 of reproduction and organs of motion may be, 

 would actually be distributed over the whole earth 

 through its multiplication in geometrfcal progres- 

 sion and its consequent migrations, if it were not 

 confined by such barriers, and had not its ranks 

 thinned by enemies and by the absence of the con- 

 ditions of existence. The various causes have 

 acted in former times just as they are acting at the 

 present day, and their combined effects and mutual 

 action and reaction are expressed in the present 

 geographical distribution of animals. 



In his admirable work on the Geographical Dis- 

 tribution of Animals Wallace adopts six great 

 regions^ in which the animals are grouped in a 

 special manner. Three of these belong to the Old 

 World, two to America, and one to Australia. 

 Each of these regions has besides a certain number 

 of sub-regions. The great Paiaearctic region 

 comprises the whole of the Eurasian continent, 

 except the south-east, together with the islands of 

 Japan, Iceland, Great Britain, the Azores, the Can- 

 ary Islands, and the islands of the Mediterranean. 

 It comprises the whole of Europe, Africa as far as 

 the Sahara and the Cataracts of the Nile, also Asia 

 Minor, Arabia, and the entire continent of Asia as 

 far as the large mountain chains of the Himalayas 

 — an enormous region, in which Wallace endeavours 

 to distinguish a European, a Mediterranean, a Sibe- 

 rian, and a Manchoorian sub-region. The great 

 iCthiopian region comprises the African con- 

 tinent south of the Sahara, and in addition to that, 

 as a sub-region, the island of Madagascar. The 



' They are adopted with certain modifications from a division of the 

 earth originally proposed by Mr. P. L. Sclater for birds, — Tr. 



great Oriental region embraces Asia to the south 

 of the Himalayas, together with the Sunda Islands 

 and the Philippines. The Australian region is 

 not restricted by Wallace to the large island of 

 Australia with Tasmania, but extends also over all 

 the islands from Celebes to the Sandwich group. 

 South America, with Mexico, Guatemala, and the 

 Antilles, form the Neotropical region, and the 

 rest of North America finally constitutes the sixth, 

 the Nearctic region. 



Of these regions, as of all others that have been 

 adopted, it may be said that none of them is limited 

 by precise boundaries, even if we leave out of ac- 

 count the more or less cosmopolitan animals, and 

 devote our attention only to forms confined within 

 narrow limits. If we would represent these regions 

 on maps, we must surround each of them with a 

 pretty extensive zone in which the forms have 

 intermingled or passed from one region into the 

 other. Besides there are in this scheme areas 

 which have been ranked as sub-regions, and which 

 yet deserve a separate independent position, at 

 least in respect of their mammalian forms. 



Thus we must undoubtedly adopt a Circum- 

 polar region, embracing the north of Siberia, 

 Lapland, Greenland, the Hudson's Bay Territories, 

 and all the islands adjacent to these portions of the 

 mainland. In all parts of this region are found the 

 same, or at any rate very closely allied, species: to 

 it belongs the territory of the polar bear, the rein- 

 deer, the glutton, the lemming, and other charac- 

 teristic forms. The island of Madagascar is totally 

 different from the mainland of Africa in respect of 

 its mammalian fauna, and, in general, is one of the 

 best characterized regions. The Antilles also 

 have scarcely anything in common with the neigh- 

 bouring mainland. If these three regions are 

 separated off as of equal value with the great 

 leading regions above named, we then have nine 

 regions marked off for the geographical distribu- 

 tion of mammals, and each of these is distinguished 

 by a separate assemblage of animals, by its own 

 peculiar fauna. 



In order to understand the partly strange partly 

 uncertain boundaries of these regions we must have 

 recourse to the indications furnished, on the one 

 hand, by geology as to the relations between land 

 and water in earlier epochs, and, on the other hand, 

 by pala;ontology concerning the existence and dis- 

 tribution of mammals which lived in those epochs. 

 In many cases these indications are still very incom- 

 plete, in others more or less uncertain. In such 



