Evolution / by Patrick Geddes and J. Arthur Thomson.

  • Patrick Geddes
Date:
[1911?]
Catalogue details

Licence: Public Domain Mark

Credit: Evolution / by Patrick Geddes and J. Arthur Thomson. Source: Wellcome Collection.

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    of the land-snail Cerion in the Bahamas, and Guliek records over 200 species of the land- snail Achatinella in the various valleys of the Sandwich Island Oahu. Many evolutionists—Wagner, Weismann, Guliek, Romanes, Jordan, and others—have worked at the idea of Isolation, as a directive factor in evolution; and Romanes maintained that it was a sine qua non in the origin of new species. The term must not be thought of in any narrow sense; it includes all the means which restrict the range of intercrossing within a species: geographical barriers, such as arise when a peninsula becomes an island; temporal barriers, such as arise when the members of a species reach sexual maturity at different times of year; habitudinal barriers, when a species splits into two or more castes with different habits of life; physiological barriers, such as arise by some variation in the reproductive organs; and psychological barriers, which rest on profound antipathies. What probably happens is this: a success¬ fully vigorous and adaptive species spreads; the several contingents become isolated from one another; and, if different variations spring up in several or all of the contingents, then, other things being equal, isolation will favour the origin of distinct species. “ I