124 THE PROBLEMS OF EVOLUTION 



The operation of the forces of selection and isola- 

 tion in nature, therefore, is due to a complex as- 

 sociation of organic functions and environmental 

 conditions which tend to accomplish one of two 

 ends: the splitting up of species into groups of 

 individuals which may differ in heritage within 

 the original limitations of the species, or the pres- 

 ervation of individuals falling within a certain 

 range of variation and the concomitant destruction 

 of the remainder. It seems wholly logical that some 

 degree of evolutionary transformation may be ac- 

 complished in nature in this way. Certainly the 

 changes wrought in domestic animals through 

 selective breeding show that possibility, even 

 though they do not demonstrate survival values 

 with respect to natural environmental relation- 

 ships. 



Another problem arising from these factors is 

 the incidence of new conditions upon the organism 

 and the reactions which they arouse. Whatever 

 may cause the isolation of a group of individuals, 

 they can scarcely meet just the same conditions 

 that have previously constituted the environment 

 of their kind. The possibilities are several. Dis- 

 persal may, as has been noted, be a result of the 

 natural tendency of a successful species to spread, 

 or it may be due to the overcrowding due to over- 

 production which Darwin regarded as so important. 

 In the former case the organism is likely to seek 



