Chap. XII. 
FRESH-WATEE PRODUCTIONS. 
383 
CHAPTEE XIL 
Geogkaphical Disteibution— continued . 
Distribution of fresh-water productions—^On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial 
Mammals — On the relation of the inhabitants of islands to 
those of the nearest mainland — On colonisation from the nearest 
source with subsequent modification — Summary of the last and 
present chapters. 
As lakes and river-systems are separated from each 
other by barriers of land, it might have been thought 
that fresh-water productions would not have ranged 
widely within the same country, and as the sea is ap¬ 
parently a still more impassable barrier, that they 
never would have extended to distant countries. But 
the case is exactly the reverse. Not only have many 
fresh-water species, belonging to quite different classes, 
an enormous range, but allied species prevail in a 
remarkable manner throughout the world. I well re¬ 
member, when first collecting in the fresh waters of 
Brazil, feeling much surprise at the similarity of the 
fresh-water insects, shells, &c., and at the dissimilarity 
of the surrounding terrestrial beings, compared with 
those of Britain. 
But this power in fresh-water productions of ranging 
widely, though so unexpected, can, I think, in most 
cases be explained by their having become fitted, in 
a manner highly useful to them, for short and fre¬ 
quent migrations from pond to pond, or from stream to 
stream; and liability to wide dispersal would follow 
from this capacity as an almost necessary consequence. 
We can here consider only a few cases. In regard to 
