76 ISLAND LIFE 



is some as yet unknown mode by which their eggs are 

 safely, though perhaps very rarely, conveyed from island 

 to island. Examples of their peculiar distribution will be 

 given when we treat of the fauna of some islands in which 

 they abound. 



The Disjjersal of Am'phibia and Frcsh-iuaUr Fishes. — 

 The two lower groups of vertebrates, Amphibia and fresh- 

 water fishes, possess special facilities for dispersal, in the 

 fact of their eggs being deposited in water, and in their 

 aquatic or semi-aquatic habits. They have another ad- 

 vantage over reptiles in being capable of flourishing in 

 arctic regions, and in the power possessed by their eggs of 

 being frozen without injury. They have thus, no doubt, 

 been assisted in their dispersal by floating ice, and by that 

 approximation of all the continents in high northern 

 latitudes which has been the chief agent in producing the 

 general uniformity in the animal productions of the globe. 

 Some genera of Batrachia have almost a w^orld-wide dis- 

 tribution ; while the tailed Batrachia, such as the newts 

 and salamanders, are almost entirely confined to the 

 northern hemisphere, some of the genera spreading over 

 the whole of the north temperate zone. Fresh-water 

 fishes have often a very wide range, the same species 

 being sometimes found in all the rivers of a continent. 

 This is no doubt chiefly due to the want of permanence in 

 river basins, especially in their lower portions, where 

 streams belonging to distinct systems often approach each 

 other and may be made to change their course from one 

 to the other basin by very slight elevations or depressions 

 of the land. Hurricanes and water-spouts also often carry 

 considerable quantities of water from ponds and rivers, 

 and thus disperse eggs and even small fishes. As a rule, 

 however, the same species are not often found in countries 

 separated by a considerable extent of sea, and in the 

 tropics rarely the same genera. The exceptions are in 

 the colder regions of the earth, where the transporting power 

 of ice may have come into play. High ranges of moun- 

 tains, if continuous for long distances, rarely have the 

 same species of fish in the rivers on their two sides. 

 Where exceptions occur, it is often due to the great 



