34 ILLINOIS STATE ACADEMY OF SCIENCE 



restricted the presentation of evolution. When the only 

 method was inference from observed facts, there was no 

 limit to inference, and it could be made to include the 

 whole plant and animal kingdoms. Now, however, the 

 experimental method limits us to a few generations, and 

 the wide-ranging inferences are left to the unscientific 

 who are not particular about the facts. 



In considering the relative merits of these explana- 

 tions, it is not necessary to subscribe to a belief in any 

 one of them, to the exclusion of the others. All of them 

 may be factors in evolution, and it is altogether probable 

 that no one of them is adequate to explain all evolution- 

 ary changes. We need them all, and more besides. 



A good method of evaluating these explanations and 

 any others that may be offered is to realize the questions 

 any explanation of evolution must answer. There are 

 at least four conspicuous questions: (1) What is the 

 cause of variation ? (2) What is the nature of the varia- 

 tions that are important in evolution? (3) How may 

 variations be perpetuated and mutiplied? (4) How are 

 the variations manipulated to effect progressive evolu- 

 tion? 



Lamarck's explanation goes farther than any other in 

 answering the first question, the cause of variation, and 

 also in suggesting a basis for progressive evolution. Dar- 

 win and DeVries accept the variations without attempt- 

 ing to explain the cause, differ as to the kind of varia- 

 tions used, and agree as to the method of manipulating 

 them. The hybridization explanation answers the third 

 question, how variations are perpetuated and multiplied. 

 It will be noted that no one of them answers all these 

 questions. 



Such an estimate of the proposed explanations empha- 

 sizes the fact that there must be more exact experimental 

 evidence before much further progress can be made in 

 solving the problems of evolution. It was in realization 

 of this that at the beginning of the present century the 

 study of evolution culminated in, and became diverted 

 into, genetics, the experimental study of inheritance, 

 which has already suggested many things, and promises 

 to be still more suggestive in the future. 



