Origin of Species Variorum

is small in comparison with the number of species inhabiting an equal area of land, the competition will probably be less severe between aquatic than between terrestrial species; consequently an intruder from the waters of a foreign country will have a better chance of seizing on a new place, than in the case of terrestrial colonists. We should also remember that .. many fresh-water productions are low in the scale of nature, and .. we have reason to believe that .. low beings change or become modified less quickly than the high; and this will give a longer time than the average for the migration of the same aquatic species. We should not forget the probability of many species having formerly ranged as continuously as fresh-water productions ever can range, over immense areas, and having subsequently become extinct in intermediate regions. But the wide distribution of fresh-water plants and of the lower animals, whether retaining the same identical form or in some degree modified, I believe mainly depends on the wide dispersal of their seeds and eggs by animals, more especially by fresh-water birds, which have great powers of flight, and naturally travel from one ... piece of water to another. ↑