Geographical Distribution continued Summary and Analysis

It would seem, on the theory that all organisms belonging to the same species or other classification originated from a single location, that freshwater organisms would generally be confined to relatively small areas. Rivers within a country are separated from one another by land and rivers in distant countries are separated by saltwater oceans, an environment in which freshwater creatures cannot generally survive.

This occurrence may be explained by a certain adaptation possessed by many freshwater creatures that allows them to migrate from one river to another or from one pond to another. In the case of fish, there is no known case of the same species of freshwater fish existing on different continents, a fact which would be impossible to explain if it were true. However, these species of fish are frequently found in other rivers...