28 
DISPERSAL AND MIGRATION. 
[part i. 
larvae 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 which 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. 
