l6 INTRODUCTORY. [CHAP. 



migrations when, as in the Old World, their trend is from west to 

 east, than when, as in America, they run from north to south, and 

 thus do not interfere with the free movements of plain-dwelling 

 animals on either side. In many instances the mammals inhabit- 

 ing each of the isolated mountain-chains, as those of Europe, are 

 to a great extent specifically identical one with another, not having 

 had time to become modified into distinct species since they 

 reached their present haunts at the close of the glacial period. 

 Even among these, however, there are indications of the com- 

 mencement of specific differences, the chamois of the Caucasus 

 forming a variety differing somewhat from the typical Alpine race. 

 Where the isolation has been longer, as in the case of the fauna 

 of the highlands of Tibet, the difference is much more strongly 

 marked ; the mammalian fauna in this instance being as peculiar 

 and distinct as that of many ancient continental islands. 



Probably ever since man has existed in any numbers on the 

 influence of glbe he has been exerting a more or less strongly- 

 ManonDistri- marked influence on the distribution of animals, 

 either by destroying them, or by conveying them to 

 countries or districts which are not their natural home. By the 

 involuntary aid of man the common rat and mouse, which belong 

 to a genus unknown in the New World, have been conveyed to 

 every country in the globe ; while the rabbit has been carried to 

 the Antipodes, where it has flourished and increased in an 

 unprecedented manner. Cattle and horses have been introduced 

 into South America, Australia, and other countries where they 

 were naturally unknown, and by their rapid increase have shown 

 that the absence of particular animals from particular districts is 

 not necessarily due to their being unsuited to live there, but rather 

 to the fact that they have been unable to find their way thither. 

 The fallow-deer, again, has been imported from its Mediterranean 

 home into England and other countries of northern Europe; while 

 goats and pigs have been carried to a number of oceanic islands, 

 where they have done irreparable harm in exterminating the native 

 fauna and flora. 



In all these instances the fact of the introduction has always 

 been more or less clearly known, and therefore no difficulty arises 

 as to what are native and what are introduced forms. Very 



