228 THE BOUNDARY-LINE BETWEEN GEOLOGY AND HISTORY, 



All these clianges produced by man affect only single species, and 

 not tbe whole fauna. The phenomena which must be ascribed to geo- 

 logical revolutions are much grander. The flora of the Canary Islands 

 and of the Azores, in particular, shows so great a resemblance to that 

 of the Western European coasts, that we must assume the former con- 

 nection of these points in spite of their present distance apart. The 

 inhabitants of the island of Madagascar differ, on the other hand, from 

 those of the eastern coasts of Africa, and those of the Galapagos 

 Islands from those of the coasts of South America. Hence it follows 

 either that the separation of these islands from the continent is older 

 than the inhabitants of the islands, or older than those of the conti- 

 nent, or older than either. 



In Central Europe there are to day two remarkable examples of di- 

 vided regions. The first consists in the identity of the fish species in 

 our various rivers, and this is at least partially explained, on geologi- 

 cal grounds, by the very plausible supjjosition of a large stream in the 

 region of the present North Sea, which had the Thames, the Elbe, and 

 others for tributaries. The present inhabitants of our rivers may be 

 considered as the isolated remains of those which formerly- i)eopled the 

 great united stream. 



The second phenomenon is the following : On the isolated heights of 

 various mountains a peculiar flora repeats itself, and many species of 

 this Alpine flora are found again far away in Scandinavia and Lapland. 

 Many animal species are distributed in the same way. The white 

 mountain-hare, {Lepus variahiUs,) for instance, is found in the pine-dis- 

 tricts of the Alps, on the mountains of Scotland and Ireland, and in 

 Scandinavia, Lapland, Northern Eussia, Siberia, and Greenland. If 

 this animal came on our mountains from the far north, how does it hap- 

 pen that it is not found in the intervening valleys ? 



Now if the hypothesis of the original connection of such regions is 

 correct, these Alpine species must have had some connection with the 

 northern ones ; and since it has been observed that the reindeer and 

 lemming not always lived iar north, but also in Central Europe, and that 

 the marmot could also exist there, it is highly probable that in Central 

 Europe all those species of plants and animals existed then which are 

 now found both on our mountains and on those in the north. 



In the colder time these beings, therefore, had their common abode in 

 Ceiitral Europe, and were distributed gradually while the change of 

 temperature was going on, since they could only find the conditions 

 necessary to their existence on high mountains or in boreal countries. 

 Some only remained in the valleys, (those of the second and third 

 groups,) some became extinct, (those of the first group,) and some emi- 

 grated, (those of the fourth group.) At the same time new species of 

 animals and j^lants appeared, which form the greater part of those of 

 the present day. The merit of having indicated how we may obtain 



