EVOLUTION 175 



ancient periods. It was ushered in by the work of 

 DeVries, who introduced the experimental study of 

 evolution, and announced his explanation of evolution 

 by means of mutation. The problem was to discover 

 whether one species actually produces another one. It 

 had been inferred that it does, but inference is not 

 demonstration. By means of carefully controlled pedi- 

 gree cultures, DeVries discovered a plant in the actual 

 performance of producing occasionally a new form 

 among its numerous progeny. This form bred true and 

 preserved its distinctive characters; in other words, it 

 was a new species, or at least a different species from 

 its parent. Many such species have now been ob- 

 served originating in this way, both in plants and 

 animals. That one species can produce another one is 

 no longer inferred, but demonstrated, and demonstrated 

 repeatedly. There is no longer any doubt, therefore, 

 that evolution is a fact. It is quite a different question 

 whether the proposed explanations are adequate. 



This outline of methods and results in one phase of 

 one science is illustrative of all scientific investigation. 

 It is uncovering facts by experimental demonstration, 

 and is taking less account of inferences. In the field 

 of evolution, when inferences were the only results, it 

 was natural to extend inference to the evolution of the 

 plant and animal kingdoms, and this involved the origin 

 of man. In these days, there is no such attempt, for 

 experimental demonstration of the evolution of the 

 whole series of organic forms, culminating in man, is 

 clearly impossible. Biologists, therefore, are no longer 

 concerned with the whole story of evolution, but only 



