564 Mutations 



incompleteness of heredity, but in vain. The 

 succeeding generations, if produced from true 

 representatives of the new type, and with pure 

 fertilization, have repeated the splitting in the 

 same numerical proportions. The instability 

 seems to be here as permanent a quality as the 

 stability in other instances. Even here no se- 

 lection has been adequate to change the original 

 form. 



IV. Some of the new strains are evidently ele- 

 mentary species, while others are to be consid- 

 ered as retrograde varieties. 



It is often difficult to decide whether a given 

 form belongs to one or another of these two 

 groups. I have tried to show that the best 

 and strictest conception of varieties limits 

 them to those forms that have probably 

 originated by retrograde or degressive steps. 

 Elementary species are assumed to have been 

 produced in a progressive way, adding one 

 new element to the store. Varieties differ from 

 their species clearly in one point, and this is 

 either a distinct loss, or the assumption of a 

 character, which may be met with in other 

 species and genera. LaevifoUa is distin- 

 guished by the loss of the crinkling of the 

 leaves, hrevistylis by the partial loss of the 

 epigjmous qualities of the flowers, and nanella is 

 a dwarf. These three new forms are therefore 



