THE THEORY OF ORGANIC EVOLUTION 181 

 Wallace's chart of natural selection 



PROVED FACTS CONSEQUENCES 



A Rapid Increase of Numbers Struggle for Existence 

 B Total Numbers Stationary 



C Struggle for Existence Survival of the Fittest 



D Variation and Heredity (Natural Selection) 



E Survival of the Fittest Structural Modifications 



F Change of Environment 



The importance of Darwin's work in the history of 

 scientific thought is that it convinced science of the truth 

 of organic evolution and proposed a plausible theory of 

 evolutionary causation. Since Darwin's time, evolution has 

 received confirmation on every hand. It is now regarded by 

 competent scientists as the only rational explanation of an 

 overwhelming mass of facts. Its strength lies in the extent 

 to which it gives meaning to so many phenomena that 

 would be meaningless without such an hypothesis. 



But the case of natural selection is far different. Of 

 recent years, this theory of the causes of evolution has 

 suffered a decline. No other hypothesis, however, has com- 

 pletely displaced it, and it remains the most satisfactory ex- 

 planation of the origin of adaptations, although its all- 

 sufficiency is no longer accepted. Moreover, the initial step 

 in evolution is the appearance of individual variations which 

 are perpetuated by heredity, rather than the selection of 

 variations after they have appeared. The interest of in- 

 vestigators has now shifted to problems of variation and 

 heredity. 



As a result of this situation, there has been much discus- 

 sion among scientists regarding the adequacy of what is 

 often referred to as the Darwinian Theory, meaning Natural 

 Selection. In condemning selection as an inadequate ex- 

 planation of the problem, biologists have often seemed to 



