118, the danger of intercrossing is reduced to
a minimum.
Several writers, however, not content with the indirect effects of
isolation here indicated, maintain that it is in itself a cause of
modification, and ultimately of the origination of new species. This
was the keynote of Mr. Vernon Wollaston's essay on "Variation of
Species," published in 1856, and it is adopted by the Rev. J.G. Gulick
in his paper on "Diversity of Evolution under one Set of External
Conditions" (_Journ. Linn. Soc. Zool._, vol. xi. p. 496). The idea seems
to be that there is an inherent tendency to variation in certain
divergent lines, and that when one portion of a species is isolated,
even though under identical conditions, that tendency sets up a
divergence which carries that portion farther and farther away from the
original species. This view is held to be supported by the case of the
land shells of the Sandwich Islands, which certainly present some very
remarkable phenomena. In this comparatively small area there are about
300 species of land shells, almost all of which belong to one family (or
sub-family), the Achatinellidae, found nowhere else in the world.
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