56 Mutability and Individual Variation. 



same tree. And Davenport and Blankinship have 

 recently shown in a valuable paper that in the case of 

 Typha latifolia and augustifolia the curves describing 

 their various characters overlap. A small leaf of latifolia 

 can be smaller than the broadest leaf of augustifolia and 

 so forth. Tlie curves overstep the limits between two 

 species; they are transgressive and the species become 

 ''inter grading groups:'^ 



The differences between the single species of Draha 

 vcrna (Fig. 3) afford one of the best examples for 

 making clear, in a general way, the nature and size of 

 mutations. 



§ 5. THE ELEMENTS OF THE SPECIES. 



Ever since Darwin's theory of descent obtained gen- 

 eral recognition, the need of an experimental study of 

 the origin of species has always been strongly felt. This 

 demand was always kept in the forefront by the few 

 opponents of this theory, who objected that, so long as it 

 was not possible to produce new species, or at least ob- 

 serve their origin directly, the foundation on which the 

 theory rested was one of unproved h3^pothesis. 



In the discussion of this objection two entirely differ- 

 ent things are usually confounded. The origin of species 

 is not the same thing as the origin of specific characters. 

 The former is a historical occurrence ; the latter is a 

 physiological one. How, when and where species exist- 

 ing at the present moment arose is a subject for historical 

 investigation, and we can only discover anything definite 

 about it in those rare cases in which records have been 

 kept by contemporary eye-witnesses. The problem of 



^ C. B. Daxtnport and J. W. Blankinship, A Precise Criterion 

 of Species; Science, N. S., Vol. H, No. 177, p. 685, 1808. 



