No. 493] ISOLATION AND SELECTION 



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organisms existing at the saute time. In certain cases the 

 isolative process may be selective at the same time; and 

 so demarcational and intensive segregation progress to- 

 gether ; as is seen when the strong and courageous migrate 

 together, leaving the weak and timid in the old habitat. 



DlVEESITY IN THE USE OF A COMPLEX ENVIRONMENT 



An illustration of the wide divergence that may take 

 place in a single family distributed over a small area is 

 found in the Achatinellidse, a family of snails found only 

 on the Hawaiian Islands. Some genera of this family are 

 found only on the ground, others chiefly on the trunks and 

 branches of the trees, others on the leaves of the trees and 

 shrubs. Ages of divergent evolution have made the indi- 

 viduals of each genus entirely incapable of crossing with 

 those of other genera; but in the case of closely related 

 varieties and species occupying different species of trees 

 in the same valley the conditions are very different. 



be attributed either to external barriers or to physiological 

 incompatibilities. The different methods of using the 

 environment, whether due to habits or to inherited charac- 

 ters, is the real cause of the isolation. Again, of the ten 

 genera of this family some are confined to one or two 

 islands, though the vegetation and other conditions sur- 

 rounding the family are much the same on the seven 

 islands of the group. The condition explaining the small 

 area occupied by any one of the five hundred species of 

 this familv is the limited power and opportunity for migra- 

 tion or transportation; and the great variety of types pre- 

 sented is undoubtedly due, first, to the power of variation 

 possessed by isolated branches of the same species while 

 using like environments in the same way; and second, to 

 the variation in isolated groups introducing divergent 

 methods of using the same complex environment and so 

 subjecting themselves to divergent forms of selection, 

 even when no external barriers hold them apart. 



In Science for March 30, 1906, Dr. Ortmann refers to 

 species of crawfish subjecting themselves to diverse con- 



