The geographical distribution of animals, with a study of the relations ...

chap. II.] REPTILES AND FISHES. 29The amphibia are much less sensitive to cold than are truereptiles, and they accordingly extend much farther north, frogsbeing found within the arctic circle. Their semi-aquatic lifealso gives them facilities for dispersal, and their eggs are no doubtsometimes carried by aquatic birds from one pond or stream toanother.Salt water is fatal to them as well as to their eggs, andhence it arises that they are seldom found in those oceanicislands from which mammalia are absent. Deserts and oceanswould probably form the most effectual barriers to their dispersal;whereas both snakes and lizards abound in deserts, andhave some means of occasionally passing the ocean which frogsand salamanders do not seem to possess.Means of Dispersal of Fishes.—The fact that the same speciesof freshwater fish often inhabit distinct river systems, provesthat they have some means of dispersal over land. The manyauthentic accounts of fish falling from the atmosphere, indicateone of the means by which they may be transferred from oneriver basin to another, viz., by hurricanes and whirlwinds, whichoften carry up considerable quantities of water and with it fishesof small size. In volcanic countries, also, the fishes of subterraneanstreams may sometimes be thrown up by volcanic explosions,as Humboldt relates happened in South America.Anothermode by which fishes may be distributed is by their eggs beingoccasionally carried away by aquatic birds ; and it is stated byGmelin that geese and ducks during their migrations feed on theeggs of fish, and that some of these pass through their bodieswith their vitality unimpaired. 1Even water-beetles flying fromone pond to another might occasionally carry with them some ofthe smaller eggs of fishes.are also enabled toBut it is probable that fresh-water fishmigrate by changes of level causing streamsto alter their course and carry their waters into adjacent basins.On plateaux the sources of distinct river systems often approacheach other, and the same thing occurs with lateral tributarieson the lowlands near their mouths. Such changes, althoughsmall in extent, and occurring only at long intervals, would1Quoted in LyelPs Principles of Geology (11th ed. vol. ii. p. 374), fromAmozn. Acad. Essay 75.