356 Harper: Native weeds and their probable origin 



\ 



intervals between which were marked by great and rapid changes 

 on the earth's surface.* Another circumstance which points to 

 the same conclusion is that closely related species do not usually 

 occupy the same habitat, as every experienced field worker knows. f 

 An extremely significant point is that no case of mutation 

 seems to have hitherto been detected in any species whose envir- 

 onment has not been disturbed by civilization. All plants now 



I 



being studied which give promise of mutation phenomena are 

 either weeds or cultivated plants. Probably no species can come 

 into close contact with civilization for any length of time without 

 being more or less modified sooner or later. As Dr. H. J. 

 Webber said about ten years ago : J "No instance is known of 

 a plant being long under cultivation and not furnishing several 

 varieties/ ' 



Most of our introduced weeds, or rather their ancestors, doubt- 

 less experienced the same change of environment in the Old World 



* 



centuries ago that our native weeds have in more recent times; 

 and the first step in the economic development of every one of our 

 cultivated plants which is no longer known in the wild state must 

 have been the creation of a new environment for it, purposely or 

 otherwise. 



Against the comparatively sudden origination of new forms 

 under the pressure of changed environment might be urged the 

 often cited experiments of European botanists who have trans- 

 planted alpine plants to lowlands and vice versa without producing 

 any permanent or inheritable variations thereby. The answer to 

 this is that probably such experiments have not been continued 

 long enough. For all we know, a century or more of exposure to 

 different conditions may be necessary in most cases to start muta- 

 tion, or " break the type," as Dr. Webber puts it. Some species 

 are doubtless more susceptible than others, § and those expen- 



* See the papers by Dall, King, LeConte, and White, cited below. 



fSee in this connection Abrams, Science II. 22 : 836-838. 22 D I9°5- 



t Yearbook U. S. Dept. Agr. 1896: 103. 1897. ., k . 



§ This might be inferred from the fact that in the foregoing list such large fan" 

 as Gentianaceae, Ericaceae, Umbelliferae, Orchidaceae, and Liliaceae are noi J^^ 

 at all, and some others very sparingly in proportion to their numbers. Wood} P ^. 

 must change more slowly than herbs, and this is perhaps one reason why the specie 

 Crataegus and Kubus have not been greatly multiplied by systematists until lately- 



