278 COOK 



that large numbers of organisms advance in company, as in 

 specific groups A species is an organization of diverse, inter- 

 breeding individuals, dependent for its continued existence upon 

 its ability to maintain a broad and intricately interwoven net- 

 work of descent. 



Speciation is the attainment of differential characters by seg- 

 regated groups of organisms, that is, by subdivisions of older 

 species. 



Isolation of an organic group implies such a separation that 

 interbreeding with members of other groups is excluded. 



Isolation is of primary importance in speciation, since isolated 

 groups of organisms always become different, but there is no 

 indication that isolation is an evolutionary factor in the sense of 

 causing or contributing to organic development. Its influence 

 is negative rather then positive, for small groups of individuals 

 advance less rapidly then large, and often deteriorate through 

 inbreeding and inadequate diversity of descent. 



The multiplying of species is a process distinct from develop- 

 mental progress, and constitutes a distinct scientific problem. 



Evolution might be explained without explaining speciation, 

 and speciation without explaining evolution. Recognition of 

 the diversity of the problems enables the factors to be separated ; 

 evolution depends upon symbasis, speciation upon isolation. 



The segregation of a new group, whether by geographic 

 barriers or by selective discrimination, merely affords opportu- 

 nity for a new evolution to go forward. The means by which the 

 progress is accomplished are to be sought inside the group, and 

 not in the mere fact of isolation or selection. The multiplica- 

 tion of the number of evolving groups is a phenomenon distinct 

 from that of the evolution itself. The evolutionary question is 

 not how the species become isolated, but how they become dif- 

 ferent after they have been isolated. 



Adaptation is the attainment of characters which place the 

 species in a more advantageous relation with its environment. 



Selection is a form of isolation which eliminates from the spe- 

 cies individuals lacking in the expression of certain characters. 



Under unconscious or natural selection only the most deficient 

 in these characters are rejected ; under conscious or artificial 



