1868.] HOOKER — ARCTIC FLORA. 359 



both are arctic and alpine plants, but S. cemua is considered by 

 some botanists to be an alpine form of the lowland S. granulata, 

 whose limits and distribution are very difficult to settle, because 

 it apparently passes into several oriental forms, which have been 

 distinguished as species. In this case I have not included S. 

 granulata with S. cemua ; because the latter is everywhere easily 

 distinguished as a well-marked plant, having a restricted range 

 both in area and elevation, which S. granulata does not share. 

 At the same time I am in favour of a hypothesis that would give 

 these a common origin previous to the glacial epoch. 



Other reasons for adopting the system of including very closely 

 allied species are the following : — When species have been founded 

 in error ; this generally arises from their authors having imperfect 

 specimens, or too limited a series of them ; various species founded 

 by Brown on the first Arctic American collections come under 

 this category, as do Adams's Arctic Siberians pecies ; the genera 

 Ranunculus, Draba, Armaria, and Potcntilla, offer many 

 examples : When the species, besides belonging to very variable 

 genera, are apparently identical both in the herbarium and 

 according to their descriptions, and present the same or a con- 

 tinuous distribution; of this Trientalis, Scnecio, Aster, Erigeron, 

 Mertensia, Sedum, Claytonia, Turritis, and many others, afford 

 examples. 



It may be asked what useful scientific results' can be obtained 

 from the study of a flora whose specific limits are in so vague a 

 condition ? the answer is, that though much is uncertain, all is 

 not so ; and that if the species thus treated conjointly really 

 express affinities far closer than those which exist between those 

 treated separately, a certain amount of definite information, useful 

 for my purpose, is obtained ; and it is a matter of secondary 

 importance to me whether the plants in question are to be con- 

 sidered species or varieties. Again-, if, with many botanists, we 

 consider these closely allied varieties and species as derived by 

 variation and natural selection from one parent form at a com- 

 paratively modern epoch, we may with advantage, for certain 

 purposes, regard the aggregate distribution of the very closely 

 allied species as that of one plant. When sufficient materials 

 shall have been collected from all parts of the arctic and sub-arctic 

 areas, we may institute afresh the inquiry into their specific 

 identity or difference, by selecting examples from physically differ 

 ing distant areas, and comparing them with others from inter 



