58 THE WANDERINGS OF ANIMALS [CH. 



extended southward to a line drawn from London to 

 Moscow, and the strip of country intervening between 

 this line and the Alps and Pyrenees was certainly 

 cold enough to support reindeer in the south of 

 France and lemmings even in Portugal, so that there 

 may easily have occurred a shifting of Scandinavians 

 to the Alps, and the reverse current of spreading 

 with the recession of the ice-sheet. Such spreading 

 and contracting with consequent isolation has no 

 doubt taken place in many parts of the world. We 

 must resort to it for the present discontinuous dis- 

 tribution of many similar, even identical, species on 

 that large system of mountains from the Pyrenees, 

 the Alps, the Caucasus, right into and beyond Central 

 Asia. 



But this same principle emphatically cannot 

 be applied to the many alpine districts which exist 

 within the tropics and are isolated from each other. 

 Here we have to remember that most of the great 

 ranges of the world are of comparatively recent date, 

 chiefly mid-Tertiary ; and as during these epochs the 

 world seems to have enjoyed a decidedly warm 

 climate, it follows that the respective faunas and 

 floras must have assumed their ' arctic facies ' after 

 the origin of these mountains and independently of 

 each other. 



