File:The variations and ecological distribution of the snails of the genus Io (1915) (14757813336).jpg

Identifier: variationsecolog00adam (find matches)
Title: The variations and ecological distribution of the snails of the genus Io
Year: 1915 (1910s)
Authors: Adams, Charles C. (Charles Christopher), 1873-1955
Subjects: Io (Gasteropoda)
Publisher: (Washington, Govt. Print. Off.)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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resencein the smooth shells may be looked upon as so much contamination from another strain withwhich they are fertile when crossed. Undulations progressively decline with departure fromthe South Fork of the Holston. The relations of these forms seem relatively simple. On theother hand, spinosity presents a much more complex problem, because it not only varies indegree, but also in the time of its appearance in the life of the animal. It may develop only latein life, or it may do so from its earHest postembryonic development; and at least several of thesedifferent degrees of spinosity are represented by local races. Some of the degrees of spinosityare dependent on the size of the stream, because in general large shells live in large streams,and large shells have large spines; but the time of life at which spinosity develops appears to bemore of a racial character. For this reason the time of life at which spines develop will be given 82 MEMOIRS NATIONAL ACADEMY OF SCIENCES. (Vol. XII.
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NO. 2.) SNAILS OF THE GENUS 10—ADAMS. 83 more weight in estimating the genetic relations than the degree of spinosity, although it mustbe borne in mind that the time of life at which spines develop may be only an expression of thedegree of intensity of the spinose character, a lesser degree of spinosity may be said to be domi-nant over a greater degree, because relatively smooth shells appear to be dominant in inversion. Turning now to the evolution of the different forms, it seems that on account of the lack ofspines, few undulations, and their occurrence in the headwaters, the smooth shells are the mostprimitive. On examination of the forms of shells which appear to stand more or less inter-mediate between the relatively smooth and those of the turrita type, as paulensis, lyttonensis,lovdonensis, and possibly verrucosa, it is readily seen that they are not a homogeneous series oftransitional forms. All are geographically located between smooth and spinose shells, withwhich they in

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