The geographical distribution of animals, with a study of the relations ...

;28 DISPERSAL AND MIGRATION. [part i.larvse have all changed into winged insects. But this favourablebreeding district will change its position with change of climateand as the last great change has been one of increased warmthin all the temperate zones, it is probable that many of the migratorybirds are comparatively recent visitors. Other changes mayhowever have taken place, affecting the vegetation and consequentlythe insects of a district ; and we have seldom the meansof determining in any particular caseextension of range occurred.in what direction the lastFor the purposes of the study ofgeographical distribution therefore, we must, except in specialcases, consider the true range of a species to comprise all thearea winch it occupies regularly for any part of the year, whileall those districts which it only visits at more or less distantintervals,apparently driven by storms or by hunger, and whereit never regularly or permanently settles, should not be includedas forming part of its area of distribution.Means of Dispersal of Reptiles and Amphibia.—If we leaveout of consideration the true marine groups—the turtles and seasnakes—reptilesare scarcely more fitted for traversing seas andoceans than are mammalia. We accordingly find that in thoseoceanic islands which possess no indigenous mammals, land reptilesare also generally wanting.The several groups of these animals,however, differ considerably both in their means of dispersaland in their power of resisting adverse conditions. Snakes aremost dependent on climate,becoming very scarce in temperateand cold climates and entirely ceasing at 62° north latitude, andthey do not ascend very lofty mountains, ceasing at 6,000 feetelevation in the Alps. Some inhabit deserts, others swamps andmarshes, while many are adapted for a life in forests. Theyswim rivers easily, but apparently have no means of passingthe sea, since they are very rarely found on oceanic islands.Lizards are also essentially tropical, but they go somewhatfarther north than snakes, and ascend higher on the mountains,reaching 10,000 feet in the Alps. They possess too someunknown means (probably in the egg-state) of passing over theocean, since they are found to inhabit many islands where thereare neither mammalia nor snakes.