318 THE INDUCTI0N3 OF LIOLOGY. 



gruous witli the hypothesis, that each species was originally 

 placed in the regions most favourable to it. But the absence 

 of a species from regions that are favourable to it, cannot be 

 thus accounted for. Were plants and animals localized wholly 

 with reference to the fitness of their constitutions to surround- 

 ing conditions, we might expect Floras to be similar and 

 Faunas to be similar, where the conditions are similar ; and 

 we might expect dissimilarities among Floras and among 

 Faunas, proportionate to the dissimilarities of their conditions. 

 But we do not find such anticipations verified. 



Mr Darwin says that " in the Southern hemisphere, if we 

 compare large tracts of land in Australia, South Africa, and 

 western South America, between latitudes 25° and 35°, we shall 

 find parts extremely similar in all their conditions, yet it 

 would not be possible to point out three faunas and floras 

 more utterly dissimilar. Or again we may compare the pro- 

 ductions of South America south of lat. 35° with those north 

 of 25°, which consequently inhabit a considerably different cli- 

 mate, and they will be found incomparably more closely related 

 to each other, than they are to the productions of Australia 

 or Africa under nearly the same climate." Still more striking 

 are the contrasts which Mr Darwin points out, between closely- 

 adjacent areas that are totally cut-off from each other. " No 

 two marine faunas are more distinct, with hardly a fish, shell, 

 or crab in common, than those of the eastern and western 

 shores of South and Central America ; yet these great faunas 

 are separated only by the narrow, but impassable, isthmus of 

 Panama." On opposite sides of high mountain- chains, also, 

 there are marked differences in the organic forms — differ- 

 ences not so marked as where the barriers are absolutely im- 

 passable ; but much more marked than are necessitated by 

 unlikenesses of physical conditions. 



Not less suggestive is the converse fact, that wide geogra- 

 phical areas which offer decided geologic and meteorologic 

 contrasts, are peopled by nearly- allied groups of organisms, if 

 there are no barriers to migration. " The naturalist in tra- 



