T>. 
90 RYDBERG: PHY APHICAL NOTES 
Europe, as for instance Potentilla nivea, Lloydia serotina and Viola 
biflora? This question leads to other related ones. Has a species 
originated only once or can the same plant arise at the same time 
or at different times at two or more isolated localities? Are the 
individuals of Potentilla nivea, now growing in Colorado, in the 
Alps, in Scandinavia, in Greenland, etc., offsprings of the same 
parent Potentilla nivea living ages ago, or did the species originate 
independently at the different places? In the case of Potentilla 
nivea, I rather think that it is monotypic and has had a much 
more general distribution than it now has, for it is a common plant 
in the arctics. But this is not the case with Viola biflora, which is 
not an arctic plant. Until lately the only localities known in this 
country were in Colorado, but now it is known also from Alaska. 
It is not, however, impossible that some so-called species have 
had a polytypic origin. Primula farinosa is a plant of the Alps, 
northern Europe, northeastern America, and apparently the same 
plant is found in south Chile to Terra del Fuego. In both cases, 
however, the plant itself or else the parent plant, from which it 
originated, must have had a much greater distribution than it 
now has. We know of many plants which in earlier periods have 
been distributed over much greater territory than they now are, 
as for instance Ginkgo biloba and Taxodium distichum were once 
found in Europe, while they now are restricted, the former to 
northeastern Asia and the latter to the southern United States. 
It was stated before that many of the alpine plants of the Rockies 
are also arctic or subarctic, and as far as the Rockies are con- 
cerned the seeds could be carried by wind and animals from moun- 
tain top to mountain top, as the stretches are not so far, the moun- 
tain chain running north and south, but this would not be a 
satisfactory explanation in Europe and Asia where the principal 
chains run east and west and at a great distance from the arctic. 
Another explanation must therefore be given. The most plausible 
and most generally accepted theory concerning the origin of the 
alpine flora is the following. In the glacial periods, the regions 
in front of the ice-sheet became unfit for the woody vegetation and 
in fact for all plants except the arctic-alpine elements. The 
forest receded south in front of the advancing ice. The temperature 
in the mountain regions south thereof became lower and lower, the 
