SUMMARY OF EVIDENCE 87 



producing new species by a manipulation of nature, 

 it seems reasonable to believe that such results will 

 occur when nature is left to itself, provided sufficient 

 time is given. 



9. The most unmistakable direct evidence of nat- 

 ural evolution comes from observed instances of natural 

 mutation — from the facts, that is, which have been 

 employed in support of the mutations theory of the 

 origin of species by discontinuous variation. What- 

 ever may be their value as evidences of this explanation 

 of species forming, they certainly constitute actual 

 instances of the origin of species by natural evolution. 

 These instances are indeed few, and Hugo de Vries' 

 explanation of this is that species are not in a mutable 

 state except for brief periods, recurring at rare inter- 

 vals. In order, therefore, to bring mutations under 

 scientific observation, species must be found that have 

 reached a mutable stage. Some years since de Vries 

 discovered that certain plants of the evening primrose, 

 Oenothera lamarckiana, growing in Holland, were 

 throwing off new varieties. Taking these plants under 

 careful observation he found that some of them gave 

 birth to new species which remained constant and bred 

 true without the assistance of artificial selection or 

 cross-breeding. His observations were verified by 

 T. H. Morgan, and by Professor MacDougal, and have 

 had considerable influence upon scientific opinion. 

 De Vries maintains that the permanent new breeds 

 which are supposed to be produced by artificial selec- 

 tion really owe their origin to natural mutation or 



