Harper : Native weeds and their probable origin 357 



merited with might have happened to be among the least 



susceptible. 



Another important point to be considered is that if our native 

 weeds have sprung into existence in modern times, they should all 

 be closely related to species that are undoubtedly native. This 

 indeed seems to be true in most cases; and the rest deserve 

 further investigation. It seems very likely that in this class of 

 plants we may have many examples of polygenesis, i. e., of a single 

 species originating independently at several or many different 

 places ; a phenomenon whose possibility was denied by Darwin 

 and some of his successors.* 



The suggestion that mutation may be induced by civilization 

 is not exactly a new one, for premonitions of it have appeared in 

 one form or another in the writings of botanists and zoologists 

 several times in the last few years, f and one occasionally hears 

 interesting rumors about it from persons who have not ventured to 

 put their ideas on the subject into print ; but it does not seem to 

 have attracted much attention yet, especially among systematists, 

 many of whom are going right on describing "new species" from 

 unnatural habitats without inquiring into their history or making 

 any distinction between them and species which are truly indig- 

 enous to our primeval forests. X 



To admit the modern origin of native weeds would simplify a 

 number of vexed problems, and put an end to many of the fruitless 

 discussions of the indigeneity of certain species which have been 

 going on in some of our botanical journals.! It would also partly 

 explain the greater richness of the herbaceous flora of Great 



*See Clements, Bot. Surv. Neb. 7 : 68-77. *9<H J Research methods in ecology 



2 3o-2 32 . 1905. 



tSee Dunn, Jour. Bot. 40: 359. 1902; Spalding, Plant World 10: 140. Je 

 'W; Cockerell, Science II. 26 : 72-73. 19 Jl I9<>7- 



t As recently as August, 1 907, a prominent systematist has published the following 

 statement (Jour. Bot. 45 I 290) : "The origin of these many forms [of Crataegus in 

 North America] I cannot pretend to account for. The theory that they are hybrids of 

 recent origin, however, can hardly be accepted." 



\ In one magazine alone, the Journal of Botany, I have noticed discussions of this 



f ( mos tly relating to British plants) at the following places in the last twelve 



flumes: 34: 201-204. 1896; 37: 356-358. 1899 5 4<> : 356. 1902; V ■ 141-H2. 



*°6 154, 285, 289-290. 1903; 43 : 89-94. 1905; 44= 138-H2, 207-213, 39°- 



