Geographical Distribution. 245 



fact which admits of no dispute, that in each of these 

 cases we meet with a direct correlation between the 

 kind of barrier and the kind of organisms whose 

 structural affinities are affected thereby. Where we 

 have to do with marine organisms, barriers such as 

 the Isthmus of Panama and the varying depth of the 

 Western Pacific determine three very distinct faunas, 

 ranging north and south in closely parallel lines, and 

 under corresponding climates. Where we have to do 

 with fresh-water organisms, we find that a mountain- 

 chain only a few miles wide has more influence in 

 determining differences of organic type on either side 

 of it than is exercised by even thousands of miles of 

 a continuous land-area, if this be uninterrupted by 

 any mountains high enough to prevent water-fowl, 

 whirlwinds, &c., from dispersing the ova. Again, 

 where we have to do with terrestrial organisms, the 

 most effectual barriers are wide reaches of ocean ; 

 and, accordingly, we find that these exercise an 

 enormous influence on the modification of terrestrial 

 types. Moreover, we find that the more terrestrial 

 an organism, or the greater the difficulty it has in 

 traversing a wide reach of ocean, the greater is the 

 modifying influence of such a barrier upon that type. 

 In oceanic islands, for example, many of the plants 

 and aquatic birds usually belong to the same species 

 as those which occur on the nearest mainlands, and 

 where there are any specific differences, these but 

 rarely run up to generic differences. But the land- 

 birds, insects, and reptiles which are found on such 

 islands are nearly always specifically, and very often 

 generically, distinct from those on the nearest main- 

 land although invariably allied with sufficient close- 



