Geographical Distribution. 219 



faunas range northward and southward in parallel 

 lines not far from each other, under corresponding 

 climates " : they are, however, " separated from each 

 other by impassable barriers, either of land or open 

 sea" : and it is in exact coincidence with the course of 

 these barriers that we find so remarkable a differen- 

 tiation of the faunas 1 . Obviously, therefore, it is 

 impossible to suggest that this correlation is accidental. 

 Altogether many thousands of species are involved, 

 and within this comparatively limited area they are 

 sharply marked off into three groups as to their 

 natural affinities, and into three groups as to their 

 several basins. Hence, if all these species were 

 separately created, there is no escape from the con- 

 clusion that for some reason or another the act of 

 creation was governed by the presence of these 

 barriers, so that species deposited on the Eastern 

 shores of South America were formed with one set of 

 natural affinities, while species deposited on the 

 Western shore were formed with another set ; and 

 similarly with regard to the third set of species in the 

 third basin, which, extending over a whole hemisphere 

 to the coast of Africa without any further barrier, 

 nowhere presents, over this vast area, any other case 

 of a distinct marine fauna. But what conceivable 

 reason can there have been thus to consult these 

 geographical barriers in the original creation of specific 



The only exception is in the case of the fish on each side of the 

 Isthmus of Panama, where about 30 per cent, of the species are identi- 

 cal. But it is possible enough that at some previous time this narrow 

 isthmus may have been even narrower than at present, if not actually 

 open. At all events, the fact that this partial exception occurs just 

 where the land-barrier is so narrow, is more suggestive of migration 

 than of independent creation. 



