It is assumed by some writers that the breeding-place of a species is to be considered as its true home rather than that to which it retires in winter; but this can hardly be accepted as a rule of universal application. A bird can only breed successfully where it can find sufficient food for its young; and the reason probably why so many of the smaller birds leave the warm southern regions to breed in temperate or even cold latitudes, is because caterpillars and other soft insect larvæ are there abundant at the proper time, while in their winter home the {28}larvæ 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 consequently 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 reptiles are also generally wanting. The several groups of these animals, 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.