No. 610] MUTATION THEORY AND SPECIES-CONCEPT 



ica, M. bifolium in Europe and M. dilatatum in western 

 America and northwestern Asia. M. canadense differs 

 from M. bifolium chiefly in leaf-shape and in being typi- 

 cally glabrous. The pubescence probably was lost at one 

 stroke, just as numerous glabrous varieties arise. An- 

 other step is sufficient to account for the alteration in 

 leaf -form, so that two steps are ample for the transition 

 from one species to the other. M. dilatatum resembles 

 M. hifolium except for its relatively gigantic size and the 

 fact that it is glabrous like M. canadense. Again two 

 definite steps are sufficient to account for its origin. 



Turning now to the geographic variations of M. cana- 

 dense particularly as regards pubescence, a detailed study 

 shows that over the greater part of its extensive range it 

 is absolutely glabrous, but that pubescence has appeared 

 especially in two localized parts of its range. A heavily 

 hirsute variety interius Fernald, occurs in the Black Hills 

 of South Dakota, an exceedingly arid region on or near 

 the western extremity of the range of the species. This 

 variety is apparently restricted in its distribution to the 

 arid portion of western South Dakota, and the most rea- 

 sonable interpretation appears to be that it has originated 

 here through a marked variation, and has thus enabled 

 the species to extend its range into this arid region. 

 Further east, chiefly in Minnesota and Wisconsin, a semi- 

 pubescent form occurs, and this may be the form from 

 which the much more marked hirsute variety arose. The 

 evidence, when closely examined, favors then a discon- 

 tinuous rather than a continuous manner of origin of this 

 heavily pubescent condition. The condition itself is 

 nevertheless an adaptation, enabling the plant to survive 

 in extreme conditions of aridity. 



The monotypic genus Kruhsea is related to Streptopus 

 in much the same way that Maiantliemiim is related to 

 Smilacina. It was originally described in the "Flora 

 ■Rossica" by Ledebour as Smilacina streptopoides from 

 eastern Siberia. His name indicates his idea of its rela- 

 tionships. Baker afterwards, from plants without flowers 



