chap, v.] 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 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 larvse 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, 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 tlie 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 
