OCR Text |
Show 66 ISLAND LIFE. (I'AH'I' I. central plains and the vast ranges of the Rocky Mountains ancl Sierra Nevada, that we can hardly expect to find species whose areas may be divided maintaining their identity. Towards the north however the above-named barriers disappear, the forests being almost continuous from east to west, while the mountain range is broken up by passes and valleys. It thus happen~ that most species of birds which inhabit both the eastern and western coasts of the North American continent have maintained their continuity towards the north, while even when differentiated into two or more allied species their areas are often conterminous or overlapping. Almost the only bird that seems to have a really discontinuous range is the species of wren, Thryotho1·us bewiclcii, of which the type form ranges from the east coast to Kansas and Minnesota, while a longer-billed variety is found in the wooded parts of California and as far north as Puget Sound. If this really represents the range of the species there remains a gap of about 1,000 miles between its two disconnected areas. Other cases are those of the greenlet, Vireosylvia gilvus, of the Eastern States, and its variety, V. swainsonii, of the Western; and of the purple red-finch, Oarpodacus purpureus, with its variety 0. californicus; but unfortunately the exact limits of these varieties are in neither case known, and though each one is characteristic of its own province, it is possible they may somewhere become conterminous, though in the case of the red-finches this does not seem likely to be the fact. In a later chapter we shall have to point out some remarkable cases of this kind where one portion of the species inhabits an island; but the facts now given are sufficient to prove that the discontinuity of the area occupied by a single homogeneous species, by two varieties of a species, by two well-marked sub:. species, and by two closely allied but distinct species, are all different phases of one phenomenon-the decay of ill-adapterl, and their replacement by better-adapted forms, under the pressure of a change of conditions either physical or organic. We may now proceed with our sketch of the mode of distribution of higher groups. Distribution and Antiquity of Families.-Just as genera are CHAP. IY.] EVOLUTION THE KEY TO DISTRIBUTION. (37 groups of allied species distinguished from all other groups by some well-marked structural characters, so f amilies are groups of allied genera distinguished ~y more marked and more important characters, which are generally accompanied by a peculiar outward form and style of colouration, and by distinctive habits and mode of life. As a genus is usually more ancient than any of the species of vvhich it is composed, because during its growth and development the original rudimentary species becomes supplanted by more and more perfectly adapted forms, so a family is usually older than its component genera, and during the long period of its life-history may have survived many and great terrestrial and organic changes. Many families of the higher animals have now an almost world-wide extension, or at least range over several continents; and it seems probable that all families which have survived long enough to develop a considerable variety of generic and specific forms have also at one time or other occupied an extensive area. Discontinuity a proof of Antiquity.-Discontinuity will therefore be an indication of antiquity, and the more widely the fragments are scattered the more ancient we may usually presume the parent group to be. A striking example is furnished by the strange reptilian fishes forming the order or sub-order Dipnoi, which includes the Lepidosiren and its allies. Only three or four living species are known, and these inhabit tropical rivers situated in the remotest continents. The Lepidosinn paradoxa is only known from the Amazon and some other South American rivers. An allied species, Lepidosiren annectens, sometimes placed in a distinct genus, inhabits the Gambia in West Africa, while the recent discovery in Eastern Australia of the Ceratodus or mud-fish of Queensland, adds anot.her form to the same isolated group. Numerous fossil teeth, long known from the Triassic beds of this country, and also found in Germany and India in beds of the same age, agree so closely with those of the living Ceratodus that both are referred to the same genus. No more recent traces of any such animal have been discovered, but the Carboniferous Ctenodus and the Devonian Dipterus evidently belong to the same group, while in North America the Devonian rocks have yielded a gigantic allied form which F2 |