28 DISPERSAL AND MIGRATION. [part i. 



larvse have all changed into winged insects. But this favourable 

 breeding district will change its position with change of climate ; 

 and as the last great change has been one of increased warmth 

 in all the temperate zones, it is probable that many of the migratory 

 birds are comparatively recent visitors. Other changes may 

 however have taken place, affecting the vegetation and conse- 

 quently the insects of a district ; and we have seldom the means 

 of determining in any particular case in what direction the last 

 extension of range occurred. For the purposes of the study of 

 geographical distribution therefore, we must, except in special 

 cases, consider the true range of a species to comprise all the 

 area winch it occupies regularly for any part of the year, while 

 all those districts which it only visits at more or less distant 

 intervals, apparently driven by storms or by hunger, and where 

 it never regularly or permanently settles, should not be included 

 as forming part of its area of distribution. 



Means of Dispersal of Reptiles and Amphibia. — If we leave 

 out of consideration the true marine groups — the turtles and sea- 

 snakes — reptiles are scarcely more fitted for traversing seas and 

 oceans than are mammalia. We accordingly find that in those 

 oceanic islands which possess no indigenous mammals, land rep- 

 tiles are also generally wanting. The several groups of these ani- 

 mals, however, differ considerably both in their means of dispersal 

 and in their power of resisting adverse conditions. Snakes are 

 most dependent on climate, becoming very scarce in temperate 

 and cold climates and entirely ceasing at 62° north latitude, and 

 they do not ascend very lofty mountains, ceasing at 6,000 feet 

 elevation in the Alps. Some inhabit deserts, others swamps and 

 marshes, while many are adapted for a life in forests. They 

 swim rivers easily, but apparently have no means of passing 

 the sea, since they are very rarely found on oceanic islands. 

 Lizards are also essentially tropical, but they go somewhat 

 farther north than snakes, and ascend higher on the mountains, 

 reaching 10,000 feet in the Alps. They possess too some 

 unknown means (probably in the egg-state) of passing over the 

 ocean, since they are found to inhabit many islands where there 

 are neither mammalia nor snakes. 



