CHAP, Il.] REPTILES AND FISHES, 99 
The amphibia are much less sensitive to cold than are true 
reptiles, and they accordingly extend much farther north, frogs 
being found within the arctic circle. Their semi-aquatic life 
also gives them facilities for dispersal, and their eggs are no doubt 
sometimes carried by aquatic birds from one pond or stream to 
another, Salt water is fatal to them as well as to their eggs, and 
hence it arises that they are seldom found in those oceanic 
islands from which mammalia are absent. Deserts and oceans 
would probably form the most effectual barriers to their dis- 
persal ; whereas both snakes and lizards abound in deserts, and’ 
have some means of occasionally passing the ocean which frogs 
and salamanders do not seem to possess. 
Means of Dispersal of Fishes—The fact that the same species 
of freshwater fish often inhabit distinct river systems, proves 
that they have some means of dispersal over land. The many 
authentic accounts of fish falling from the atmosphere, indicate ' 
one of the means by which they may be transferred from one 
river basin to another, viz., by hurricanes and whirlwinds, which 
often carry up considerable quantities of water and with it fishes 
of small size. In voleanic countries, also, the fishes of subter- 
ranean streams may sometimes be thrown up by volcanic explo- 
sions, as Humboldt relates happened in South America. Another 
mode by which fishes may be distributed is by their eggs being 
occasionally carried away by aquatic birds; and it is stated by 
Gmelin that geese and ducks during their migrations feed on the 
eggs of fish, and that some of these pass through their bodies 
with their vitality unimpaired.’| Even water-beetles flying from 
one pond to another might occasionally carry with them some of 
the smaller eggs of fishes. But it is probable that fresh-water fish 
are also enabled to migrate by changes of level causing streams 
to alter their course and carry their waters into adjacent basins. 
On plateaux the sources of distinct river systems often approach 
each other, and the same thing occurs with lateral tributaries 
on the lowlands near their mouths. Such changes, although 
small in extent, and occurring only at long intervals, would 
? Quoted in Lyell’s Principles of Geology (11th ed. vol. ii. p. 374), from 
Amen. Acad. Essay 75. 
