AEEAS OF HABITATION". 21 



mountain plateaus, and at a third the grassy savannas and rolling 

 plains. Naturally, in the case of such animals as are dependent 

 for their existence upon certain physical peculiarities of their en- 

 vironment, or upon particular conditions of food and climate, we 

 shall meet mth local areas scattered through the region of dis- 

 tribution of a given species where no individuals of that species 

 are to be met with, an apparent discontinuity being thus pre- 

 sented. For instance, such denizens of the forest as the South 

 American monkeys and the sloths will but very exceptionally be 

 found anywhere else than in their forest homes, and, therefore, the 

 partial destruction of this forest, or its invasion by a grassy savan- 

 na, will tend to render the "home " of those animals discontinuous. 

 Possibilities of such, or a similar, discontinuity may likewise arise 

 in the case of the animals of the plains, marshes, and deserts, since 

 the physical aspects of the earth's surface are constantly subjected 

 to vicissitudes of greater or less magnitude, and, as a matter of 

 fact, we find numerous instances where, in an extensive range, 

 particular animals are restricted in their habitats to certain favoured 

 spots or localities. But in all or most of such instances a former, 

 and comparatively recent, continuity of area, or possibility of mi- 

 gration from one locality to another, can be proved. The chamois, 

 whose range embraces the entire east and west extent of Southern 

 Europe, is found almost exclusively on the higher mountain sum- 

 mits — the Pyrenees, Alps, Carpathians, Caucasus, and the moun- 

 tains of Greece — and would appear, therefore, to occupy several 

 widely-removed habitats. But there can be no reasonable doubt 

 that the peculiar distribution of this animal is the outcome of 

 migration from a central home. The hippopotamus is found in 

 the Nile, Niger, Senegal, and most of the larger rivers of South 

 Africa, between which stretch vast areas where no individuals of 

 the animal have ever been found — regions untenantable by reason 

 of their aridity; but here, as in the case of the chamois, there can 

 be no doubt that a migration or diffusion did take place at a time 

 when the physical aspects of the country were favourable for sucli 

 a dispersion, and were, consequently, different from what they 

 are at present. One of the most remarkable instances of areal dis- 

 continuity among mammals is that exhibited by the variable hare, 

 whose home, in the Old World, is Eurasia north of the fifty-fifth 

 parallel of latitude. The animal reappears, after skipping the low- 



