Harper : Native weeds and their probable origin 355 





all species now living originated by hybridization. But although 

 the occurrence of natural hybrids has been demonstrated in some 

 of the genera listed and strongly suspected in others,* hybridi- 

 zation would hardly increase the number of species ten-fold in 

 Crataegus for instance in so short a time. And some of the genera 



very 



limited in number. 



For the large number of cases still unexplained we shall prob- 

 ably have to fall back on the mutation theory, as being the only 

 other one which explains the sudden origin of new forms. Only 

 a few well-authenticated cases of mutation have hitherto been de- 

 tected, and there is still considerable diversity of opinion as to the 

 cause of the phenomenon. De Vries seemed to think that each 

 species passes through periods of mutation at long intervals, and 

 that his epoch-making discovery was largely due to the fact that 

 he happened to have a mutating species of Oenothera under care- 

 ful observation just at the critical period. MacDougal has suc- 

 ceeded in inducing mutation, or something that looks very much 

 like it, in the same genus by artificial means, and from this he 

 infers that the cause of mutation may at least in some cases be 

 external to the plant. f Blaringhem seeks this external cause in 

 mechanical injuries (such as cutting the stem) at the time of 

 maximum vegetative activity. % This seems quite plausible in 

 view of the fact that de Vries's Oenotheras are said to have grown 

 ln a field which was cut over sometimes, and most of our meadow 

 and roadside plants get similar treatment every year. 



Although the writer does not claim to be familiar with the 

 Physiological aspects of the mutation theory or theories, it seems 

 to him perfectly reasonable to suppose that change of environment 

 alone may be a sufficient cause, or stimulus, of mutation, or at least 

 of rapid evolution. Indeed it is more than likely that all evolu- 

 tlQ nary changes (other than hybridization) in the vegetable king- 

 dom have been preceded by changes of environment. Geological 

 h,s tory confirms this view, for one of its most prominent features 



to f L - .»«,!_ 



th « great difference in the organisms of successive periods, the 



*See MacDougal, Bot. Gaz. 43 : 57. 1907. 

 TSee Yearbook, Carnegie Inst. 5 : 129. 1907 

 X Mutation et Traumatismes. Paris, 1907. 



