ASPECTS OF KINETIC EVOLUTION 277 



called speciation, is brought about by isolation, and is not a 

 necessary cause nor a necessary result of evolution. 



In a similar way another group of evolutionary writers have 

 confused evolution with adaptation. Evolution results, not un- 

 commonly, in the production of characters which give a species 

 a specialized fitness for some particular environment. From 

 such facts it was argued that the increase of fitness or " survival 

 of the fittest" represented the method of evolution, or in other 

 words, that evolution is merely a process of adaptation actuated 

 by the selective power of the environment. The facts of nature 

 show, however, that evolutionary motion is not at all restricted 

 to directions of fitness, and it is also obvious that an evolution so 

 restricted could not produce even the characters of fitness upon 

 which it would depend for its supposed power to transform 

 species. Fitness must be attained by evolution before the envi- 

 ronment can give the character selective specialization by limit- 

 ing the evolutionary motion and deflecting it into more definitely 

 adaptive directions. 



Evolution is the process of change by which the members of 

 an organic group become different from their predecessors, or 

 from other groups of common origin. 



Symbasis is the normal evolutionary condition of free and ex- 

 tended interbreeding among the individual members of natural 

 species. 



Symbasis implies adequate diversity of descent ; it is to be 

 distinguished on the one side from the narrow inbreeding which 

 induces abnormal mutations, and on the other from the wide 

 cross-breeding which produces abnormal hybrids. 



The continual interweaving of the lines of descent from diverse 

 and unrelated ancestors appears to be necessary to sustain the 

 vitality and evolutionary progress of the higher plants and ani- 

 mals. The constructive evolution of new organic types does 

 not take place on simple or narrow lines of descent, but requires 



descent. When considered with reference to each other, the contemporaneous 

 species of a horizon have the same significance as species of the present day, but 

 species of different horizons may have a relation which two simultaneous species 

 would never have, that is, one may be the true ancestor of the other. The same 

 word species is used for several categories of organic groups. See, Four Cate- 

 gories of Species, American Naturalist, April, 1899. 



