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Show [CHAP. XI. STRIBUTION. GEOGH.APHICAL Dl 304 d' t' ct with hardly a fish, shell, marine faunas are more lh Ill 'f the eastern and western or crab in common, than t osle oAmerica yet these great shores of S ou th an d Cenbt ra the narrow' ' ·b ut l· mpassab. le , faunas are separated only y d f the shores of Amenca, isthmus of P anama. W estwat r dos with not an I·S 1 a n d as a a wide space of.open ?cean =~ ~re' we have a barrier of halting-place for. emigrant 'th's is passed we meet in the another kind, and as soopn a~fi 1 with another and totally eastern . 1 ds of the aCI c, . f IS an h three manne aunas range distinct fauna. So tha~ erd in parallel lines not far from far northward and sout war d!nO' climates. but from being each other, under c~rr~~~n b~ i'mpassabl~ barriers, either separated from eac o the:, are wholly distinct. On the of land or open se;, rn further westward from the other hand, procee Ing s .I 1 arts of the Pacific, we eneastern islands of the trbopl~a P<and we have innumerable counter no 1· mp~ssab le arners 1 nth after travelling over a islands as halting-p acesthu hores of Africa; and over hemisphere we come to ith ~o well-defined and distin~t this vast space we ft~et h hardly one shell, crab or fish IS marine faunas. . A ~u~ed three approximate faunas. of common to the above nA _. and the eastern Pacrfic Eastern and Western merf~~m the Pacific into the Inislands, yet many fish ra~~!s are common to the eastern dian Ocean, aw ~flny s d ~he eastern shores of Africa, on islands of the aCI c. an ·idians of longitude. almost e~actly oppfsl\e mei tly included in the foregoing A third great ac ~ parf the roductions of the same state.ments, is the :ffin~;~e speci~s themselves are dis~inct continent or se~, t ou£ t t' It is a law of the wid~st at different points an s a l~_ns.nt offers innumerable In· generalityN, and t~vf:ls t~: ~~~uralist in travelling, for ibnstances. ever e h never fails to be struck by t e stance, from north to sou~ ' ups of beings specifically manner in which successive gr~ h otlie~. He hears distinct, yet cle~rly rela~dti~lt k1:a:~f birds, notes near· - from closely alhed, y~t. lS t similarly constructed, but ly similar, a~d sees. t mr nes sloured in nearly the sa:rp.e not quite ahke, ":Ith eggs hco St . ts of Magellan are In· manner. The plams near t e rai OBAP. XI.] GEOGRAPHICAL DISTRIBUTION, 305 habited by one sp~cies of Rhea (.American ostrich), and northward the plains of La Plata by another species of the same genus ; and not by a true ostrich or emeu, like those found in .Africa and .Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of Rodents, but they plainly display an .A.rrwrican type of structure. We ascend the lofty peaks of the Cordillera and we :find an alpine species of bizcacha ; we look to the waters, and we do not :find the beaver and musk-rat, but the coypu and capybara, rodents of the .American type. Innumerable other instances could be given. If we look to the islands off the .American shore, however much they may differ in g~ological structure, the inhabitants, though they may be all peculiar species, are essentially .American. We may look back to past ages, as shown in the last chapter, and we find .American types then prevalent on the American continent and in the American seas. We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions. The naturalist must feel little curiosity, who is not led to inquire what this bond is. This bond, on my theory, is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like, or, as 've see in the case of varieties, ~early like each other. The dissimilarity of the inhabI~ ants of different regions may be attributed to modificatiOn through natural selection, and in a quite subordinate ~egree to the direct influence of different physical conditiOn~. The degree of dissimilarity will depend on the migratwn of the more dominant forms of life from one region Into another having been effected with more or less ease, at periods more or less remote ;-on the nature and number ~f th~ former immigrants ;-and on their action and reactwn, .In their mutual struggles for life ;-the relation of organism to organism being, as I have already often hlha~ked, the most important of all relations. Thus the g Importance of barriers comes into play by checking |