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 different. 11 

 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 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 Achatinella. 12 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 



