Periodic Mutations 689 



testing of native plants on the hypothesis of 

 unit-characters as deduced from Darwin's 

 Pangenesis. This conception led to the expec- 

 tation of two different kinds of variability, one 

 slow and one sudden. The sudden ones known 

 at the time were considered as sports, and 

 seemed limited to retrograde changes, or to 

 cases of minor importance. The idea that 

 sudden steps might be taken as the principal 

 method of evolution could be derived from the 

 hypothesis of unit-characters, but the evidence 

 might be too remote for a starting point for ex- 

 perimental investigation. 



The success of my test has given proof to the 

 contrary. Hence the assertion that no evidence 

 is to be considered as inadequate for the pur- 

 pose under discussion. Sometime a method of 

 discovering, or of producing, mutable plants 

 may be found, but until this is done, all facts of 

 whatever nature or direction must be made use 

 of. A very slight indication may change for- 

 ever the whole aspect of the problem. 



The probabilities are now greatly in favor of 

 our finding out the causes of evolution by 

 a close scrutiny of what really happens in na- 

 ture. A persistent study of the physiologic 

 factors of this evolution is the chief condition 

 of success. To this study field-observations 

 may contribute as well as direct experiments, 



