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- 
