174 



The Origin of Species by Mutation. 



ing with all the elementary forms that exist must be the 

 duty of monographs of a greater degree of completeness.^ 

 When it is a question of the origin of one elementary 

 species from another this material is absolutely essential 

 to the student of evolution. When however our object 

 is the study of the relationship of the larger groups it 

 certainly constitutes a mere dead weight, the fact of 

 whose existence one is only too often tempted to sup- 

 press. But I can see no rea- 

 son why these two branches 

 of inquiry should not exist 

 side by side. Nothing but 

 a belief in the supernatural 

 value of the Linnean species 

 can stand in the way. 



In the natural state it iS 

 only very rarely that ele- 

 mentary species are distin- 

 guished by a single or by 

 two or three characters- 

 (Fig. ?)2>) ; they usually dif- 

 fer in all their organs and 

 characters. A complete diagnosis often requires a whole 

 page. The tout ensemble of the plant is quite distinctive ; 

 and the practised eye can recognize the various forms 

 at a distance.^ This is especially so in the case of cul- 



^ Compare, for example, the Flora Europae of M. Gandoger, 

 which gives the elementary forms for all the more important species, 

 but only refers to their characters in short dichotomous tables {27 

 vol.). 



'On the heaths near Amsterdam there are to be found three 

 forms of Potcntilla Tormentilla, one with narrow, one with broad 

 and one with intermediate petals ; I saved seeds from each of these 

 forms and found them to be constant. 



^ Or they may be quite or nearly indistinguishable externally and 

 differ from one another only in fundamental physiological characters 

 as for example in the choice of hosts in the case of the Rusts — facts 



Fig. 22)- Potentilla Tormentilla, 

 with narrow, broad and inter- 

 mediate petals representing 

 three constant subspecies 

 found in nature. 



