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Show 382 GEOGRAPHICAL DISTRIBUTION. CHAP. XI. widely dispersed to various points of the southern hemisphere by occasional means of transport, and by the aid, as halting-places, of existing and now sunken . islands, and perhaps at the commencement of the Glacial period, by icebergs. By these means, as I believe, the southern shores of America, Australia, New Zealand have become slightly tinted by the same peculiar forms of vegetable life. Sir C. Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate on geographical distribution. I believe that the world has recently felt one of his great cycles of change; and that on this view, combined with modification through natural selection, a multitude of facts in the present distribution both of the same and of allied forms of life can be explained. The living waters may be said to have flowed during one short period from the north and from the south, and to have crossed at the equator; but to have flowed with greater force from the north so as to have freely inundated the south. As the tide leaves its drift in horizontal lines, though rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountainsummits, in a line gently rising from the arctic lowlands to a great height under the equator. The various beings thus left stranded n1ay be compared with savage races of man, driven up and surviving in the mountainfastnesses of almost every land, which serve as a record, full of interest to us, of the former inhabitants of the surrounding lowlands. CHAP. XII. FRESH-\V A'rER PRODUCTIONS. 383 OI-IAPTER XII. GEOGRAPHICAL DISTRIBUTION-continued. Distribution of fresh-water productions- On the inhabitants of oceanic islands- Absence of Batrachians and of terrcstr1al Mammals- On the relation of tbe inhabitants of islands to those of the ne:trest mainland- On colonisation from the nearest source with subsr.quent modification- Summary of the last and present chapters. As lakes and river-systems are separated from each other by barriers of land, it might have been thought that fresh-water productions would not have ranged widely within the same country, and as the sea is apparently a still more impassable barrier, that they never would have extended to distant countries. But the case is exactly the reverse. Not only have many fresh-water species, belonging to quite different classes, an enormous range, but allied species prevail in a remarkable manner throughout the world. I well remember, when first collecting in the fresh waters of Brazil, feeling much surprise at the similarity of the fresh-water insects, shells, &c., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain. . But this power in fresh-water productions of ranging Widely, though so unexpected, can, I think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream ; and liability to wide dispersal would follow from this capacity as an almost necessary consequence. vV e can here consider only a few cases. In regard to |
