CHAr. v.] DISPEESAL 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 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 larvae 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 Switzer- 

 land consisted largely of genera still inhabiting Europe, and 

 even of a considerable number identical, or almost so, wdth 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 hemi- 

 sphere in a somewhat more southern latitude. There is, there- 

 fore, 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 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 



