30 Mutability and Indiz'idiial Variation. 



proposition that races, varieties and subspecies of wild 

 as well as of cultivated plants have arisen by certain 

 modifications from "species" received general recogni- 

 tion. Darwin had collected all the facts available for a 

 history of these changes in the case of cultivated plants.^ 

 They provide us with a history of garden plants and 

 often also give the source and the date of introduction 

 of varieties, but they do not tell us whence they came 

 or how they arose.- 



^'Varieties arc incipient species" and ''species have 

 descended, like varieties, from other species" ; these are 

 the two famous propositions which Darwin is contin- 

 ually asserting and with whose proof he is chiefly con- 

 cerned.^ In other words: the origin of varieties from 

 species is granted ; why not species from species ? In 

 order to prove this it is obviously not necessary to know 

 the exact way in which varieties themselves originate. 

 It is sufficient tliat the relation between species and 

 genera is the same as that between varieties and species. 



Darwin asserts again and again that it must not be 

 forgotten that under the term of variations mere indi- 

 vidual differences are included.^ His variabilitv is there- 

 fore always to be understood in a double sense. It con- 

 sists on the one hand of individual difi^erences and on 

 the other of single variations.'^ The former belong to 

 those phenomena which we now term individual varia- 



^ Of later works compare especiallj' Alph. de Candolle^ Suv 

 Vorigine dcs plantes cultivccs. 



^ See also Bateson, Materials, p. 17. 



^ Origin of Species, 6th ed., pp. 2, 4, 86, etc. 



* Origin, ibid., pp. 64, 80, etc. 



^ Life and Letters, III, p. 108. As examples of single variations 

 are considered such cases as the color of the flowers of Datura Tatula 

 (a blue form belonging to the white-flowered D. Strainoniiim) and 

 the absence of spikes on the fruits of Datura mcrmis. See Fig. 5. 



