74 



ISLAND LIFE. 



[part I. 



their eggs being deposited in water, and in their aquatic or 

 semi-aquatic habits. They have another advantage 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 pro- 

 ductions of the globe. Some genera of Batrachia have almost 

 a world-wide distribution ; 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 systemxs often 

 approach each other and may be made to change their course 

 from one to the other basin by very slight elevations or depres- 

 sions 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, how- 

 ever, 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 mountains, 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 antiquity of the group, which has survived so many 

 changes in physical geography that it has been able, step by 

 step, to reach countries which are separated by barriers impass- 

 able to more recent types. Yet another and more eflicient 

 explanation of the distribution of this group of animals is the 

 fact that many families and genera inhabit both fresh and salt 

 water.; and there is reason to believe that many of the fishes 

 now inhabiting the tropical rivers of both hemispheres have 

 arisen from allied marine forms becoming gradually modified 



