488 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
eral distribution and of uniform morphological characters through- 
out their range probably accompanied the more easily differentiated 
members of the various invasions and meeting again in Scandinavia 
resumed their former continuity. Thus the pine (Pinus sylvestris) 
may have come to Scandinavia by two different routes, from the 
south and from Scotland.t. As far as I know, a thorough compara- 
tive study of the morphological characters of the Scotch and west 
Norway pines has not been made to ascertain if any (possibly quite 
minute) differences exist which distinguish them from the east 
Scandinavian pine, although from experiments made in western 
Norway with seeds from the Scotch tree there seems to be at least 
some physiological similarities between them as distinguished from 
the eastern pines. 
Slight morphological differences between Scotch and Norwegian 
forms, however, need not mean that they have no genetic relation. 
It depends upon other circumstances how such differences may. 
be interpreted. While undoubtedly many species may have retained 
their identity in spite of the segregation on both sides of the North 
Sea since glacial times, others more plastic, as the phrase is, may 
have been differentiated into diagnosable races on account of a 
similar geographical separation during the same space of time. 
Take as an example the case of the tundra ptarmigans (Lagopus 
mutus and L. rupestris) which ornithologists consider different 
species. The former extends in Scandinavia east to the White Sea, 
' the latter from there on eastwards, while in the west and south 
Lagopus mutus still lives in Scotland, the Alps and. the Pyrenees. 
At least two of these isolated colonies of the ptarmigan have be- 
come slightly differentiated, viz., the Scandinavian and the form 
inhabiting the Alps. Whether the Scotch and Pyrenean ptarmigans 
also show the effect of the long segregation is not known but 
matters very little in the present connection, since the general dis- 
tribution of the collective species is fair proof of its belonging to 
the whole assemblage of animals and plants which came to Norway 
from the west. 
However, granting that such a biota invaded Norway from the 
direction of Scotland, does it necessarily follow that it traveled 
over a continuous land bridge? 
In attempting to answer this question it is imperative to discuss 
the various categories composing the flora and fauna involved. 
* Analogous to the dual origin of the spruce (Picea) in eastern and 
northern Norway. 
