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Show 334 GEOGRAPHICAL DISTRIBUTION. [CHAP. XIL CHAPTER XII. GEOGRA. PHICAL DI,STRIBUTION-COntinued. . . th . habitants of oceanic islands-Ab. D' t 'bution of fresh-water 'productiOns-On e l~s On the relation of the inhabit- ISs ernl ce o f Batrachians andf otfh terreasrtersita lr o~aamm la mnd a- -On colonisation from the neahr est ants of islands to those o e ~e . -Summary of the last and present c ap- source Wl't h subsequent modrficatwn ters. se arated from each other As lake.s and river-~yst~m~t a~:ve ~een thought that f~·es~- by barriers of l~nd, lt ml£ not have ranged widely Within water productions w~u the sea is apparently a still more the same countr:y, an as he never would have extended impassable barri~r, th~ ~ tte case is exactly the reverse. to distant countries. fresh-water species, belonging to Not only have many rmous range but allied spe-qulte different classes, an lenbol manner throughout the ci.e s preva1'1 l·n a remba r ra he en first collecti. ng I.n th e world. I well rem~m er~ w much sur rise at the simifresh waters of Brazil, fee~Ing t shells p &c and at the . f th f esh-water 1nsec s, ' ., . ldaisnstiym iol aritye orf the s~rr~un ding terrestrial bmngs, com-pared with: those of f3rfta~h-water productions of ranging But this power In re., I think in most cases widely, though so UJ?-expe~ted,b~~~~e fitted in a manner be explained by their h£vin1 t and frequ~nt migrations highly useful to them, or ~t~~am to stream; and l!abilfrom po~d to :pond, ~r frofd follow from this capamt:y as ity to Wide dispersa wou . We can here consider an almost necessary conseque~c~. fish I believe that the only a few cases. In reg:ar tho fresh waters of distant same species never occur ~e c~ntinent the species ~ften continent·sa. But on the sa . . usly . for two river· range WI el y and almost capricio ' CB..&.P. XII.] FRESH-WATER PRODUCTIONS. 335 systems will have some fish in common and some different. A few facts seem to favour the possibility of their occasional transport by accidental means ; like that of the li~e fish not rarely dropped by whirlwinds in India, and the vitality of their ova when removed from the water. But I am inclined to attribute the dispersal of fresh-water fish mainly to slight changes within the recent period in the level of the land, having caused rivers to flow into each other. Instances, also, could be given of this having occurred during floods, without any cnange of level. We have evidence in the loess of the Rhine of considerable changes of level in the land within a very recent geological period, and when the surface was peopled by existing land and fresh-water shells. The wide difference of the fish on opposite sides of continuous mountain-ranges, which from an early period must have parted river-systems and completely prevei?-ted thei: inosculation, see~s to lead to this same conclusion. With respect to alhed freshwater fish occurring at very distant points of the ·world, no doubt there are many cases which cannot at present be explained: but ~orne fresh-water fish ~elong to very ancient forms, and In such cases there Will have been ample time for great geographical changes, and consequently time and means for much migration. In the second place, salt-water fish can with care be slowly accustomed to live in fresh water; and, according to Valenciennes, there is hardly a single group of fishes confined exclusively to fresh water, so that we may imagine that a marine member of a fresh-water group might travel far along the shores of the sea, and subsequently b~come modified and adapted to the fresh waters of a distant land. Some species of fresh-water shells have a very wide range, and allied species, which, on my theory, are descended from a common parent and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me .much, as their ova are not likely to be transported by birds, and they are immediately killed by sea-water, as are the adults. I could not ev~n understand how some naturalised epecies have |