Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 GEOGRAPHIC ISOLATION 77 while another with low vagility may be split into numerous local forms. Thus the species of birds are often represented in Germany each by a single subspecies. Certain flightless ground beetles have numerous subspecies in German}', while snails, especially aquatic snails, vary almost from station to station. With a low degree of vagility mere distance becomes a sufficient barrier to prevent inter- breeding. All the conditi
Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 GEOGRAPHIC ISOLATION 77 while another with low vagility may be split into numerous local forms. Thus the species of birds are often represented in Germany each by a single subspecies. Certain flightless ground beetles have numerous subspecies in German}', while snails, especially aquatic snails, vary almost from station to station. With a low degree of vagility mere distance becomes a sufficient barrier to prevent inter- breeding. All the conditions are favorable to the formation of local forms among the reef corals, whose vagility is restricted to a brief larval period; with the manifold conditions of their environment, their variability is very great, and even neighboring reefs may be When there are no impassable barriers, neighboring geographic forms may be connected by intermediates, as are the subspecies of the wide-ranging puma, or those of the snail Murella (Fig. 4) in 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 su
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