32 ILLINOIS STATE ACADEMY OF SCIENCE 



DeVries' explanation simply changes the type of 

 variation subjected to selection. Instead of a new species 

 being built up gradually, it is born full fledged, and nat- 

 ural selection merely decides which of the fledglings 

 shall survive. This explanation encountered the objec- 

 tion that the so-called mutating forms are simply hybrids 

 splitting. In fact, the original classic example of muta- 

 tion, Oenothera Lamarckiana, has turned out to be prob- 

 ably a hybrid, and not a genuine case of mutation. The 

 situation was concealed for a time by the fact that the 

 ratio of a splitting hybrid and the ratio shown by these 

 so-called mutants were very far from consistent. This, 

 however, has now been explained by work in genetics, 

 so convincingly, in fact, that DeVries himself has ac- 

 cepted the explanation. His attitude toward his proposed 

 explanation of evolution should be understood. He told 

 me on several occasions that he was not at all sure of 

 this explanation, but that he prided himself not on his 

 theory, but on the fact that he had started a new method 

 of studying evolution, that is the experimental method. 



I might also state for your benefit an experience I had 

 showing the same spirit in Darwin with reference to his 

 explanation. As you know, Asa Gray was the champion 

 of Darwinism in this country, writing many notable 

 papers on the subject, which were afterwards collected 

 in a volume entitled Darwimana. On one occasion Dr. 

 Gray showed to me a letter he had received from Darwin 

 after the latter had read one of these papers. In the 

 letter Darwin said: "You have stated the case so 

 clearly and convincingly that I am almost persuaded to 

 believe it myself." In other words, these pioneers in 

 evolutionary theory realized better than their followers 

 that their explanations were only tentative, to be tested 

 by subsequent investigation. They were suggestions 

 rather than conclusions, to be thought about rather than 

 believed. 



Weismann's explanation, revived by Lotsy, that 

 hybridization is responsible for evolution, encountered 

 the obstacle that although hybridizing multiplies varia- 

 tions, it can never account for original differences. It 

 results in mixtures of various kinds, but introduces 



