xviii.] DISTRIBUTION. 225 



by the agency of man into countries to which they Foreign 

 were not native, and thrivinsr so well in their new abode T,tn]Zr m-, 



■* o gciiiiing on 



that they have gained on, and partly expelled, the indi- native 



ones 



genoiis species. Thus, European thistles and clover have 

 covered great part of the Pampas of Buenos Ayres, and 

 European clover and other plants are rapidly superseding 

 the indigenous herbage of New Zealand.^ 



And there is evidence which, if possible, is still more 

 direct and conclusive. No one can doubt the purpose of 

 the webbed feet of water-fowl ; yet there are geese that Upland 

 inhabit dry places, and make no use of their webbed feet. s°°®®- 

 Nor can any one doubt that our woodpeckers' feet are Ground 

 adapted to climb trees ; yet there is a woodpecker inha- ^°°^lj. 

 biting the Pampas of South America, where trees are 

 unknown.^ The inference is obvious, and I think certain, 

 that the "upland geese" are a colony of geese which 

 have abandoned an aquatic Kfe; and that the wood- 

 peckers of the Pampas are a colony of woodpeckers 

 which have strayed away from their aboriginal forests, 

 or perhaps have been expelled by the increase of some 

 other species of animal that preyed on them. In both 

 these cases the species have become modified in their 

 new habitats. 



Another most significant fact is, that small islands at a Bats on 

 distance from any continent usually contain no indigenous [giauds. 

 mammalia whatever, except hats, and many peculiar species 

 of these are found on such islands. How have those 

 species originated ? The obvious answer is, that they are 

 the descendants of bats that were once blown across the 

 sea, and have become modified into new species in their 

 new abodes.^ 



A fact which at first sight is not quite so intelligible is, Birds on 

 that among the small number of known species of land- * ^ablT"' 

 birds that are unable to fly, a large proportion are found to fly. 

 on islands. Thus, the Mauritius, before it was colonized 

 by man, was inhabited by the dodo, an enormous sluggish The dodo, 

 bird ; and the neighbouring island of Eodriguez was inha- 



1 Darwin's Origin of Species, p. 242. 



2 Ibid. p. 212. 3 Ibid. p. 469. 



