402 THE INDUCTIONS OF BIOLOGY. 



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 

 adjacent areas that are totally cut off from each other. " Xo 

 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- 

 velling, for instance, from north to south never fails to be 

 struck by the manner in which successive groups of beings, 

 specifically distinct, yet clearly related, replace each other. 

 lie hears from closely allied, yet distinct kinds of birds, notes 

 nearly similar, and sees their nests similarly constructed, 

 but not quite alike, with eggs coloured in nearly the same 

 manner. The plains near the Straits of Magellan are inhabit- 

 ed by one species of Rhea (American Ostrich), and northward 

 the plains of La Plata by another species of the same genus ; 

 and not by a true ostrich or emu, like those found in Africa 



