. Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage . Fig. 4.—Series of forms of Helix (Murella) scabriuscula (1-2), globularis (4-8), and sicana (9-10), from the mountains of Sicily, arranged in a geographical series from west to east. After Kobelt. Sicily. The presence of such transitional forms is the most usual criterion for subspecific classification. Transitional variation may be wanting at the boundary between the ranges of vicarious forms which are then considered specifically distinct. Occasional hybridization as in the c
. Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage . Fig. 4.—Series of forms of Helix (Murella) scabriuscula (1-2), globularis (4-8), and sicana (9-10), from the mountains of Sicily, arranged in a geographical series from west to east. After Kobelt. Sicily. The presence of such transitional forms is the most usual criterion for subspecific classification. Transitional variation may be wanting at the boundary between the ranges of vicarious forms which are then considered specifically distinct. Occasional hybridization as in the crows, Corvus corone and C. comix, does not appear to corre- spond to subspecific intergradation. An excellent example of the effect of geographic isolation within a restricted area is supplied by the Achatinellidae of the Hawaiian Islands. These tree snails occur in the numerous wooded ravines which radiate from the volcanoes, and the treeless ridges between them afford an effective barrier to their distribution. Thus many of these valleys have a special subspecies, and the small island of Molokai, with an area of 720 sq. km., has 70 species and subspecies of Achatinellidae, of which 56 belong to the genus Such thorough isolation is much more common for land and fresh-water animals than for marine animals, whose continuous environment reduces the effective- ness of such barriers as exist. This is one of the reasons for the great
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Keywords: ., bookcentury1900, bookcollectionbiodive, booksubjectanimalecology
