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 systems 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 efficient 
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 
