CHAP. V DISPERSAL OF ANIMALS AND PLANTS 77 



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 for a life in fresh water. 

 By some of these various causes, or a combination of them, 

 most of the facts in the distribution of fishes can be 

 explained without much difficulty. 



The Dispersal of Insects. — In the enormous group of 

 insects the means of dispersal among land animals reach 

 their maximum. 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. The eggs and 

 larv86 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 important factor in 

 the problem is the immense antiquity of insects, and the 

 long persistence of many of the best marked types. The 

 rich insect fauna of the Miocene period in Switzerland con- 

 sisted largely of genera still inhabiting Europe, and even 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. The cause of this 

 wonderful restriction to limited areas is, undoubtedly, the 

 extreme specialisation of most insects. They have become 

 so exactly adapted to one set of conditions, that when 



