326 Science of Plant Life 



breeders is in harmony with the theory of evolution, and 

 plant breeding has furnished us suggestions as to the methods 

 by which evolution has taken place. 



Evolution proved; methods of evolution not proved. 

 Among the thousands of students of modern and ancient 

 plants, there are few, if any, who do not regard evolution itself 

 as a fact. The evidence points so clearly toward evolution, 

 and all other explanations for the origin of the plant species 

 now on the earth have proved so inadequate, that botanists 

 consider evolution as proved. At the same time, we know 

 comparatively little today concerning the agencies that have 

 brought about evolution. Evolution as a process has been 

 studied only recently, and merely a beginning has been made 

 in unraveling the tangled skein of causes and methods by 

 which modern plants have come to be what they are. 



Factors of evolution. When we seek to learn the reasons 

 for the changes in plant life during geological times, we find 

 that many factors were probably involved. We can get an 

 idea of some of the possible factors from suggestions that have 

 been made by students of evolution, but we must keep in 

 mind the fact that there is today little real evidence concern- 

 ing the factors of evolution and that the suggestions merely 

 indicate how evolution may have come about. It seems 

 probable, however, that in the production of new kinds of 

 plants variation and selection play a part. Certainly there 

 may be among the offspring of a plant individuals that differ 

 from the parent, and some variations are known to be inher- 

 itable. In order to explain evolution, it is necessary to assume 

 that new characters are produced in plants by variation, and 

 that the changes in the plants are transmitted from genera- 

 tion to generation. 



