EXTIlfrCTIOIf OF AITIMAL GEOUPS. 305 



conditions had gone on, however slowly, becoming less and less 

 favourable, we assuredly should not have perceived the fact, yet 

 the fossil horse would certainly have become rarer and rarer, and 

 finally extinct ; its place being seized on by some more successful 

 competitor." The case of the horse here stated strikingly illus- 

 trates the mystery in which the subject of extinction is still involved, 

 and how very limited is our knowledge in this direction. Why 

 the animal should have become extinct on the American continent, 

 and at so early a period after its evolution, when under apparently 

 identical, or, at any rate, very similar, physical conditions, it con- 

 tinued to develop and thrive in the Eastern Hemisphere, is a prob- 

 lem towards the solution of which we can oSer but vague conjecture. 

 Nor is the difficulty in the matter a whit lessened, but rather the 

 contrary, by the circumstance of the ready adaptability to its appa^ 

 rently unfavourable environs which the more recently introduced, 

 or modern, horse has shown. Instances of this kind, anomalies 

 to our existing knowledge, are by no means rare. The extinction 

 of the mammoth in the far north of the Eastern Hemisphere, and 

 the survival of its first cousin, the elephant, in the south, are equally 

 inexplicable. Both, as far as we are permitted to judge, appear to 

 have been in harmony with their surroundings ; vegetable-feeders, 

 they inhabited regions of sufiiciently luxurious vegetation, the one, 

 provided with a shaggy coat of hair to protect it from the rigours of 

 the frozen north, and the other, more nearly naked, suited to a 

 home where little or no protection from climatic extremes was neces- 

 sary. Both, again, were inhabitants of regions where a struggle 

 against the attacks of savage Carnivora was a part of their existence, 

 and if any advantage favoured the one side above the other in such 

 internecine warfare, it was on the side of the northern species. It 

 may just be, in the case of the mammoth, that the extreme cold 

 of the Glacial epoch, combined with a continuous submergence of 

 the land surface beneath an ice - cap, so far reduced the plant- 

 growth of the north as no longer to provide adequately for the 

 sustenance of these monsters, and that, as a consequence, they 

 gradually diminished in numbers, and eventually completely van- 

 ished. Such a supposition, however, must needs remain in the 

 nature of a mere hypothesis, until more facts than we now possess 

 shall have been gathered, indicating for it a high probability. 

 Similarly, the theory which accounts for the disappearance of the 



