27 



native rose rusts are reported, notably all the southern region, 

 and the plateau between the Sierra and Rocky Mountains. 

 Probably this is in part due to the sparseness of native hosts in 

 these areas, to the oversight of collectors, or it may be to the 

 absence or rarity of the rusts because of unfavorable conditions. 

 At present it is only possible to call the attention of observers to 

 this hiatus in our knowledge. 



The especially prominent feature brought out in the study of 

 the native rose rusts is the remarkable parallelism between them 

 and their hosts in regard to geographical distribution and specific 

 variability. Each species of rust inhabits one species of host or 

 a group of species of similar physical characteristics, and ranges 

 over quite definite areas, usually nearly coextensive with the 

 range of the respective hosts. Probably the most variable 

 species of all is P. montivaginn of the Rocky Mountains, and it 

 is also true that the roses on which it occurs form the most intri- 

 cate complex of ill-defined species known to North America. 

 Furthermore, intergrading forms are not infrequent between the 

 mountain species, P. viontivagmu, and the prairie species, P. 

 Rosae-arkansanae , along the foothills of Colorado and Wyom- 

 ing, just as intergrading forms of the hosts also occur along this 

 tension line. 



In explanation of these facts probably many of the ecological 

 factors controling the distribution of the hosts on which the rusts 

 occur would also have a bearing on the distribution of the rusts 

 themselves. It is not possible, however, to resist the impression 

 that one of the chief factors is the intimate relation between host 

 and parasite. Whatever the nature of this relationship may be, 

 and it would be difficult to define it, it permits of a certain thrifti- 

 ness of the parasite in proportion to the susceptibility of the host. 

 Any tendencies to variability in the parasite must therefore be 

 accentuated by changes in the host. That the variability in the 

 parasite does not originate through any qualities in the host 

 probably needs no proof, but has an admirable illustration in this 

 connection. Earlea speciosa is found abundantly throughout all 

 the territory and upon all the hosts inhabited by the five species 

 of Pliraguiidmui, and yet shows no marked variations, whether 



