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Show 74 ISLAND LIFE. [PAU1' I. their eggs being deposited in water, and in their aquatic or semi-aquatic habits. They h~ve. a~other . adva~tage ov~r reptiles in being capable of flounshmg m a~ctiC reg10ns, ~nd m the power possessed by their eggs of bemg f~ozen .w1tho~t injury. They have thus, no doubt, been. ass~sted m thmr 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 t~e animal productions of the globe. Some genera of Batrach1a 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 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 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 impassable 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 CT:fAl'. V.J DISPERSAL OF ANIMALS AND PLANTS. 75 for a life in fresh water. By some of these various causes or a combination of them, most of the facts in the distributi~n of fishes can be explained without much difficulty. The Dispersal of lnsects.-ln the enormous group of insects the means of dispersal among land animals reach their ma.ximum. Many of them have great powers of flight, and from their extreme lightness they can be carried immense distances by gales of wind. Others can survive exposure to salt water for many days, and may thus be floated long distances by marine currents. 'l~he eggs and larvre often inhabit solid timber, or lurk under bark or in crevices of logs, and may thus reach any countries to which such logs are floated. Another ~mportant factor in the problem is the immense antiquity of msects, and the long persistence of many of the best marked types. The rich insect fauna of the Miocene period in Switzerland consisted largely of genera still inhabiting Europe, and eve~ of a considerable number identical, or almost so, with living species. Out of 156 genera of Swiss fossil beetles no less than 114 are still living; and the general character of the species is exactly like that of the existing fauna of the northern hemisphere in a somewhat more southern latitude. There is, therefore, evidently no difficulty in accounting for any amount of dispersal among insects ; and it is all the more surprising that with such powers of migration they should yet be often as restricted in their range as the reptiles or even the mammalia. 'rhe cause of thjs wonderful restriction to limited areas is undoubtedly, the extreme specialisation of most insects. The; have become so exactly adapted to one set of conditions, that when carried into a new country they cannot live. Many can only feed in the larva state on one species of plant ; others are bound up with certain groups of animals on whom they are more or less parasitic. Climatal influences have a great effect on their delicate bodies; while, however well a species may be adapted to cope with its enemies in one locality, it may be quite unable to guard itself against those which elsewhere attack it. From this peculiar combination of characters it happens, that among insects are to be found examples of the widest and most erratic dispersal and also of the extremest |
