Retrograde Varieties 141 



present in allied species. There are a great 

 many cases however, in which the morpholog- 

 ical cause of the dissimilarity is not so easily 

 discerned. But there is no reason to doubt that 

 most of them will be found to conform to the 

 rule on closer investigation. Therefore we can 

 consider the following as the principal differ- 

 ence between elementary species and varieties ; 

 that the first arise by the acquisition of entirely 

 new characters, and the latter by the loss of 

 existing qualities or by the gain of such pecul- 

 iarities as may already be seen in closely allied 

 species. 



If we suppose elementary species and varie- 

 ties originated by sudden leaps or mutations, 

 then the elementary species have mutated in the 

 line of progression, some varieties have 

 mutated in the line of progression, while others 

 have diverged from their parental types on a 

 line of retrogression, or in the way of repetition. 

 This conception agrees quite well with the cur- 

 rent idea that in the building up of the vegeta- 

 ble kingdom according to the theory of descent, 

 it is species that form the links of the chain 

 from the lower forms to the more highly organ- 

 ized later derivatives: or otherwise expressed, 

 the system is built up of species, and varieties 

 are only local and lateral, but never of real 

 importance for the whole structure. 



