What a Special Theory Must Do 291 



We can make only theoretical and fragmentary attempts to 

 explain what happens. Therefore, there is no evolution — 

 that is, nothing happens." We may paraphrase the logic 

 of this argument into a more obvious absurdity: ** The ways 

 of God pass all understanding; the fool sayeth therefore in 

 his heart, there is no God." 



A complete theory has placed upon it a very heavy 

 burden. There have been developed numerous special 

 theories although not one of them is satisfactory in every 

 respect. 



What a Special Theory Must Do 



Any theory that attempts to explain how evolution goes 

 on must in the first place be in harmony with common sense, 

 that is, with the established principles regarding the action 

 of known forces upon known materials, and more particu- 

 larly the principles regarding the action of organic matter. 

 Thus, a complete theory must answer the significant ques- 

 tions regarding heredity and must harmonize with the 

 known facts of reproductive processes, of the germinal 

 mechanism, and of individual development. It must answer 

 the question of the sources of variation, particularly of the 

 kinds of variation that are transmissible. It must answer 

 the question of organic response which we take to be the 

 foundation of individual adjustment, and of species adapta- 

 tion, if this is indeed a distinct order of fact. 



Moreover, a complete theory of organic evolution must 

 point to processes that are observable today, even though it 

 may not present to us the actual transformation of species 

 during the period of observation. That is to say, it must 

 harmonize with the assumption of uniformity in natural 

 processes. A complete theory must be adequate to account 

 for changes recorded of the past, as well as for the existence 

 of living forms. And finally, it must explain the origin of 

 adaptations as well as the mere transformation. 



We demand of a scientific theory today that it enable 

 us to predict to some extent future findings and develop- 



