196 cook: evoIvUTion through normal diversity 



It was necessary to discard the idea of diversity being lost 

 through crossing, before it could be understood that new char- 

 acters might be preserved in natural species without the indi- 

 viduals being segregated, by selection or otherwise. Preserving 

 the adaptive variations no doubt facilitates evolution in direc 

 tions of increased fitness, but the argument from fitness is far 

 from proving that changes of characters are caused by selection. 

 We say that cold weather makes us put on overcoats, but not 

 that overcoats are made by cold weather. Species become 

 adapted through variation. Each separate group is a distinct 

 evolutionary system for developing new characters, some of 

 adaptive value and others not. How characters are originated 

 and preserved in transmission are questions that relate to the 

 mechanism of heredity, but the nature of the mechanism of 

 evolution is obvious. A very effective way of extending and 

 combining any characters that variation may afford is provided 

 by the organization of each species into a continuous fabric 

 of lines of descent, united through sexual reproduction.^ 



Endless individual diversity results from the continued de- 

 velopment and gradual diffusion of inherited characters among 

 the members of a species. To find the diagnostic characters, 

 those that are shared by all the members of one species but are 

 absent from related species, often requires very patient and per- 

 sistent work by systematists. Sometimes it is impossible to 

 determine, even from many specimens, whether one species 

 or more than one is represented. The groups must be canvassed 

 in nature to learn whether they are continuous or not, so great 

 and multifarious are the individual differences, while the general 

 similarities are obscure and difficult to state. 



With diversity accepted as a normal and general condition 

 in species, evolution is seen as a process of continous integration 

 and differentiation of characters. The two essential conditions 

 of evolutionary progress are normal diversity (heterism) and 

 free intercrossing of lines of descent (symbasis), as in natural 



•* O. F. Cook. The vital fabric of descent. Proc. Wash. Acad. Sci. 7: 301-323. 

 1906. See also, Methods and causes of evolution, U. S. Dept. Agr. Bur. PI. Ind. 

 Bull. 136. 1908. 



