550 
GEOGRAPHICAL ZOOLOGY. 
[part it. 
It has already been shown (Yol. I. pp. 209-213 and Vol. II. 
pp. 44-48) that the peculiarities of distribution of the various 
groups of insects depend very much on their habits and 
general economy. Their antiquity is so vast, and their more 
important modifications of structure have probably occurred so 
slowly, that modes of dispersal depending on such a combina- 
tion of favourable conditions as to be of excessive rarity, may 
yet have had time to produce large cumulative effects. Their 
small specific gravity and their habits of flight render them 
liable to dispersal by winds to an extent unknown in other classes 
of animals ; and thus, what are usually very effectual barriers 
have been overstepped, and sometimes almost obliterated, in 
the case of insects. A careful examination will, however, almost 
always show traces of an ancient fauna, agreeing in character 
with other classes of animals, intermixed with the more promi- 
nent and often more numerous forms whose presence is due to 
this unusual facility of dispersal, 
The effectual migration of insects is, perhaps more than in 
any other class of animals, limited by organic and physical 
conditions. The vegetation, the soil, the temperature, and the 
supply of moisture, must all be suited to their habits and 
economy ; while they require an immunity from enemies of 
various kinds, which immigrants to a new country seldom 
obtain. Few organisms have, in so many complex ways, become 
adapted to their special environment, as have insects. They are 
in each country more or less adapted to the plants which 
belong to it ; while their colours, their habits, and the very 
nature of the juices of their system, are all modified so as to 
protect them from the special dangers which surround them in 
their native land. It follows, that while no animals are so well 
adapted to show us the various modes by which dispersal may 
be effected, none can so effectually teach us the true nature and 
vast influence of the organic barrier in limiting dispersal. 
It is probable that insects have at one time or another taken 
advantage of every line of migration by wdiich any terrestrial 
