Page:Popular Science Monthly Volume 10.djvu/596

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sea in the chinks of drift-wood, and this is probably the means of their dispersal.

The inhabitants of the sea seem to have unlimited facilities for journeying, but when we remember that cold water is essential to many fishes, tropical warmth to others, and the deep sea an effectual barrier to a large number of species, it is apparent that the Atlantic may be as impassable a gulf to fishes as to land-animals. Distinct river systems are sometimes inhabited by the same species of freshwater fish, which indicates that they have some means of dispersal over land. This may be accomplished by changes of level giving rise to altered river-courses and new water-basins, to transportation of the eggs by ducks, geese, aquatic birds, and even water-beetles, and to the agency of whirlwinds and hurricanes, which carry up considerable quantities of water, and with it small fishes.

Reptiles have very limited means of dispersal. Snakes are dependent on climate, being comparatively scarce in temperate and cold regions. They entirely cease in 62° north latitude, and are not found above 6,000 feet on the Alps. They swim rivers easily, but, since they are rarely met on oceanic islands, it is inferred that they have no means of crossing the sea. Lizards are also tropical animals, though they are found higher on the mountains and farther north than are snakes. They possess some means of crossing oceans, and frequently inhabit islands where there are neither snakes nor mammalia. The amphibia extend farther north than true reptiles, frogs being found, sometimes, beyond the arctic circle. Salt-water is fatal to them, and they are probably effectually limited by deserts and oceans.

It would seem, at first, that birds are limited by no barriers, and that a study of their habits could scarcely throw any light upon the causes of animal distribution; but remarkable contrasts in the extent of their range are presented by different groups of birds. Thus, the gulls (Laridæ) and petrels (Procellaridæ) are great wanderers, a few being found, with scarcely any variation, over almost the entire globe; other species being restricted to one of the great oceans; while parrots, pigeons, and many small perching birds, are confined to islands of limited extent, or to single valleys or mountains. Some birds, such as the apteryx, ostrich, and cassowary, have no power of flight, and, of course, limited means of dispersal. The short-winged birds, such as wrens and toucans, are able to fly but a short distance, and only species endowed with great powers of flight can cross extended widths of sea. Violent gales sometimes carry small birds accidentally to foreign countries, as is shown by the large numbers of North American stragglers which reach the Bermudas. Inadequate supply of food, afforded by the vegetation of a country, oceans, and even large rivers, may serve as effectual barriers to the dispersal of birds. The presence of enemies, of either the young, the eggs, or the parent-birds, may limit the range of a species. In the Malay Archipelago pigeons are