404 HISTORICAL GEOLOGY. 



of the world (as for example in North America and 

 Europe), although they are so far separated from one 

 another and completely isolated. It must be observed, 

 however, that they are also very similar to Arctic species. 

 The explanation is found in the migrations of the glacial 

 times. At that time Arctic species were pushed south- 

 ward on both continents to the shores of the Mediter- 

 ranean in one and of the Gulf of Mexico in the other. 

 On the return of a temperate climate most of them fol- 

 lowed the retreating ice-foot back to their Arctic home ; 

 but some followed arctic conditions upward to the tops of 

 high mountains, and were stranded there in alpine isola- 

 tion till now. It is true they have been slowly changing 

 since then some in one direction, some in another in 

 accordance with a universal law in the case of isolated 

 species ; but the time has been too short to effect any 

 great changes. 



2. South Africa. Africa, south of Sahara, is inhabited 

 by two very distinct groups of mammals. The first group 

 consists of small animals of very low organization, such 

 as insectivores, but very different from those found any- 

 where else. These we shall call indigenes. The other 

 group consists of very large and highly organized ani- 

 mals, mostly also peculiar to Africa, but similar in gen- 

 eral character to those found in Eurasia, especially those 

 of Pliocene times. These we shall call invaders. Now, 

 before glacial times, Africa was isolated from the rest of 

 the world and inhabited by the indigenes only. Then 

 came the glacial elevation, opening gateways through the 

 Mediterranean and into Africa, and the glacial cold 

 driving the Pliocene mammals southward into Africa, 

 where they were shut up by the closing of the passages 

 through the Mediterranean and by the formation of the 

 Desert of Sahara. The subsequent struggles between 

 invaders and indigenes, and the effect of a new environ- 

 ment on the invaders, have greatly changed both, but 



