THE ORIGIN AND THE EXTINCTION OF SPECIES 361 



St. Helena. The original and remarkable fauna and tlora oi this 

 island had for the most part disappeared 200 years ago, through the 

 cutting down of trees in the forests, and these were later wholly 

 destroyed by the introduction of goats, which devoured all the young- 

 trees as they grew. But with the forests most of the indigenous 

 insects and birds were doomed to destruction, so that now there is 

 not an indigenous bird or butterfly to be found there ; onl}^ a few 

 terrestrial snails and beetles of the original fauna still survive. 



But it is not only on islands that a large number of species have 

 been decimated or entirely exterminated by deforestation, by the 

 introduction of plants cultivated by man and of the ' weeds ' 

 associated with these, and by the importation of domesticated animals. 

 In Central Europe not only have all the larger beasts of prey, like 

 the bear, the lynx, and the wolf, almost completely disappeared, but 

 the reindeer, the bison, the wild ox (Aurochs), and the elk have been 

 exterminated as wild animals, and in North America the buffalo will 

 soon only exist in preserved herds, if that is not already the case. Here, 

 of course, the direct interference of the all-too-powerful enemy, man, 

 has played the largest part in causing the disappearance of the species 

 referred to, but the process may give us an idea of the way in which a 

 superior animal enemy may be able gradually to exterminate a weaker 

 species where there is no attainable or even conceivable variation 

 which might preserve them from such a fate. Several of the mammals 

 which I have mentioned are not yet entirely exterminated ; even the 

 Aurochs perhaps still exists in the pure white herds preserved in 

 some British parks; but there are more instances than that of the 

 Dodo of the utter extermination of a species through human agency 

 within historic times. It may be doubtful whether the sea-otter 

 (Enhydris marina) has not been already quite exterminated because 

 of its precious fur, but it is quite certain that the huge sea-cow 

 {tthytina stelleri), which lived in large numbers in the Behring Straits 

 at the end of the eighteenth and the beginning of the nineteenth 

 centuries, was completely exterminated by sailors within a few 

 decades. 



We may therefore gain from what is going on before our ej-es, 

 so to speak, some sort of idea of the way in which the extermination 

 of species may go on even independently of man at the present time, 

 and how it must have gone on also in past ages of the earth's histor\-. 

 Migrations of species have taken place ceaselessh', although very 

 slowly, for every species is endeavouring slowly to extend its range 

 and to take possession of new territories, and thus the fauna and 

 flora of any region must have changed in the course of time, new 



