340 The Mechanism of Evolution in Leptinotaksa 



delimited or undelimited, attain increased significance and open up possibili- 

 ties such that heterogeneity in nature and the origin of natural groups may 

 become a matter of direct experimentation. No doubt methods of their pro- 

 duction will be discovered that are capable of being duplicated at any time or 

 place, and no longer will species be the plausible product of adaptations, selec- 

 tions, and survivals of the fittest that have adorned the literature of evolution 

 in the last half century. 



The geographic phases of heterogeneity must assume different aspects and 

 the problems become more directly open to analytical investigation, with regard 

 to both internal and external agents. 



For me at least the phenomena of heterogeneity in nature take on a different 

 meaning, than when viewed from the one-time aspect of " variation," its cate- 

 gories, definitions, and kinds, utile and non-utile, introducing into the subject 

 and its conceptions elements of causation impossible of interpretation from a 

 physical viewpoint. The conceptions applied in the analysis of heterogeneity 

 in this investigation offer opportunity for unlimited analytical investigation, 

 and in place of biometrics and its plausibilities there are possible determina- 

 tions of the actual productive agents, their testing, rearrangement into diverse 

 combinations, and the entire removal of the essential factors in heterogeneity 

 and evolution from the situations into which Weismann and others would 

 place them, in inaccessible bearers of characters, operating in an inacessible 

 world of their own. Hope for progress in the future lies only in analytical 

 experiment, and at present the factorial conception of the constitution and 

 evolution of organisms provides the only purely physical, working hypothesis. 



