550 GEOGRAPHICAL ZOOLOGY. [part iv. 



It has already been shown (Vol. 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 eifects. 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 

 tlie 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 aninikals, limited by organic and physical 

 conditions. The vegetation, Irhe soil, the temperature, and tlie 

 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 which any terrestrial 



