THE ORIGIN OF SPECIES 275 



derived from a common ancestor in the not far distant past. 

 In the case of plants that less closely resemble one another, it 

 is conceivable that the common ancestor has existed farther 

 back in the line of descent. 



Darwinian theory. Ever since man began to think about 

 plants he has speculated more or less as to their origin. Al- 

 though the great mass of people have always believed that 

 plants have existed unchanged from the beginning, there have 

 been people in every age to point out evidences of change, 

 and it was early suggested that plants have descended from 

 earlier and different forms through modifications of structure. 

 For many centuries proofs of this idea accumulated, but the 

 whole subject did not receive adequate treatment until Charles 

 Darwin issued his " Origin of Species " in 1859. In this book 

 was gathered a great mass of facts in support of the conten- 

 tion that all plants and animals have arisen by the slow proc- 

 esses of variation and natural selection, and to this theory of 

 organic descent the name of the Darwinian theory has come 

 to be applied. 



Mutation theory. As the study of nature has progressed, 

 many instances of evolution have been encountered that are 

 difficult to reconcile with the Darwinian theory. While the 

 main features of evolution have not been questioned, there 

 has seemed to be need of additional explanations to account 

 for the origin of certain forms which it is difficult to imagine 

 could be produced by gradual changes. These are supplied by 

 the mutation theory of Hugo De Vries. This new theory does 

 not take the place of the Darwinian theory but rather sup- 

 plements it. The new theory holds that new species do not 

 always arise from old ones by a succession of slight modifica- 

 tions, but that they may spring into being fully developed, 

 much as sports and bud variations appear among cultivated 

 plants. This theory is capable of experimental proof, and De 

 Vries has produced a number of distinct forms from a single 



