THE MIGRATIONS OF MAMMALS. 237 



quered in 1780, and here, as everywhere else, the towns were first 

 colonized, and the flat country round taken in by degrees. Villages 

 not easily reached, that is to say, not lying on river-banks, were 

 only invaded in the last decade of this century: in my boyhood it 

 was still unknown in my native village, and the black rat, now 

 being crowded out even there, held undisputed possession of many 

 places where its rival now reigns supreme. Many isolated farms 

 were only reached later, about the middle of the present century, 

 but the victorious march still goes on. Not content with having 

 discovered and conquered Europe, towards the end of last century 

 the brown rat set out on new journeys. In the sea-ports already 

 colonized, the rats swam out to the ships, climbed on board by the 

 anchor chains, cables, or any other available ladders, took possession 

 of the dark, protecting hold, crossed all seas, landed on all coasts, 

 and peopled every country and island, where its chosen protector 

 and compulsory host civilized man has founded homesteads. 

 Against our will we have helped it, or at any rate made it possible 

 for it, to carry out a greater extension of range than has been at- 

 tained by any other mammal not in subjection to man. 56 



Another remarkable illustration of wandering is afforded by the 

 souslik, a destructive rodent about the size of a hamster, belonging 

 to the family of squirrels and sub-family of marmots. Eastern 

 Europe and Western Siberia are its head-quarters. Albertus Magnus 

 observed it in the neighbourhood of Ratisbon, where it is now no 

 longer found, though it has recently appeared in Silesia. Forty or 

 fifty years ago it was unknown here, but, at the end of the forties 

 or beginning of the fifties, it appeared no one could tell whence, and 

 from that time it has pressed slowly westward. Its migrations, too, 

 have been helped by man, for, though it is not confined to cultivated 

 fields, these afford the habitat most suited to its taste. 



The same holds true of many species of mice, which extend their 

 territories as the soil is cultivated. On the other hand, man narrows 

 the possible range of many mammals by deforesting, by draining 

 marshes, and by otherwise changing the character of whole tracts of 

 country. In this way, far more than by direct persecution, he does 

 much to influence the migration of the mammals which have estab- 



