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THE AMERICAN NATURALIST [Vol. XL VIII 



mentioned, and its low index of modification may be at- 

 tributed to the same sort of imperfect knowledge as in the 

 case of Borneo and New Guinea in Table X. The con- 

 stant growth of the number of species per genus from 

 2.22 to 3.43 as the various seas and oceans are added to- 

 gether, exactly parallels the results obtained in a similar 

 way for a terrestrial group in Table XII. The comparison 

 of the speciation of the largest area for which it was 

 worked out, with the speciation of the entire group, many 

 species and genera of which inhabit fresh water, is inter- 

 esting, jumping as it does from 3.43 to 4.22. From the 

 facts brought to light by this table it can hardly be doubted 

 that practically the same influence is brought to bear on 

 the speciation of marine as on terrestrial organisms by 

 the extent of their distribution. 



The theoretical explanation here proposed for this phe- 

 nomenon involves a number of complex problems relating 

 to evolution and speciation, including isolation, effect of 

 time, causes of specific and generic modification, etc., each 

 of which will be dealt with in the following pages as they 

 seem to influence the law here proposed. 



Let us first consider the factor of isolation in relation 

 to the production of new forms. As excellently stated by 

 Cook (1909), isolation can not be considered as a cause or 

 factor in evolution, since changes in the characters of 

 species are not dependent upon the subdivision of species 

 to form additional species. To quote from him : 



The separation of species into two or more parts allows the parts 

 to become different, but there is every reason to believe that evolutionary 

 changes of the same kind would take place if the species were not 

 divided. That the isolated groups become different, does not indicate 

 that isolation assists in the process of change. It gives the contrary 

 indication that changes are restricted by isolation. If isolation did not 

 confine the new characters to the group in which they arise, the groups 

 would remain alike, instead of becoming different. . . . Isolation is 

 the shears that splits the species, not the loom that weaves it. 



Therefore, while isolation can not be considered a factor 

 in evolution, it is an important factor in speciation. 

 Species vary in many directions or orthogenetically pro- 



