57 8 THE POPULAR SCIENCE MONTHLY. 



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 fresh- 

 water 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 np considerable 

 quantities of water, and with it small fishes. 



Reptiles have very limited means of dispersal. Snakes are de^ 

 pendent 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 (Laridce) and petrels (Procellaridce) 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 par- 

 rots, 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 Ameri- 

 can 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 



