forms of life in all parts of the globe. The living waters have
flowed during one period from the north and during another from the
south, and in both cases have reached the equator; but the stream of
life has flowed with greater force from the north than in the opposite
direction, and has consequently more freely inundated the south. As the
tide leaves its drift in horizontal lines, rising higher on the shores
where the tide rises highest, so have the living waters left their
living drift on our mountain summits, in a line gently rising from
the Arctic lowlands to a great latitude under the equator. The various
beings thus left stranded may be compared with savage races of man,
driven up and surviving in the mountain fastnesses of almost every
land, which serves as a record, full of interest to us, of the former
inhabitants of the surrounding lowlands.

CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions--On the inhabitants of oceanic
islands--Absence of Batrachians and of terrestrial Mammals--On
the relation of the inhabitants of islands to those of the nearest
mainland--On colonisation from the nearest source with subsequent
modification--Summary of the last and present chapters.

FRESH-WATER PRODUCTIONS.

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 formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to different classes, an enormous
range, but allied species prevail in a remarkable manner throughout
the world. When first collecting in the fresh waters of Brazil, I well
remember feeling much surprise at the similarity of the fresh-water
insects, shells, etc., and at the dissimilarity of the surrounding
terrestrial beings, compared with those of Britain.

But the wide ranging power of fresh-water productions 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, within their own countries; and
liability to wide dispersal would follow from this capacity as an almost
necessary consequence. We can here consider only a few cases; of these,
so