STEJ NEGER] ANIMALS AND PLANTS OF NORWAY 487 
persal (p. 319). It is also to be noted that he specifically excludes 
Jederen (p. 337) and the coast south of it from the direct line of 
invasion and shows that the Arctatlantic plants reached that part 
of southwestern Norway at a much later period. 
VII. THe Way or DISPERSAL INTO WESTERN NORWAY 
It is clear that if a more or less ice-free land bridge existed be- 
tween northern Scotland and west Norway during the glacial period, 
_a whole complex biota must have crossed it. Animals and plants 
are so closely bound together that the movement of the fauna must 
necessarily depend upon that of the corresponding flora. Whether 
such an invading biota is to be composed of many or few species 
is determined by several circumstances. Of these the length of time 
during which the land connection lasted, the diversity of environ- 
ment, including climatic conditions, and the distance in space from 
the center whence the biota emigrated, are the principal factors. 
A consideration of the simple fact that some species spread much 
slower than others shows this contention to be true. Therefore, if 
we remember that part of the association of plants and animals, 
which in this paper it is suggested immigrated into western Norway 
by a Scoto-Norwegian land bridge, at one time must have had its 
center of distribution in western continental Europe before entering 
England, while a large portion of the animals are supposed to hail 
even from western Asia, it will be seen that a considerable distance 
had to be traversed and that western Norway lies at the extreme 
end of a long and tortuous route. Small wonder if the fauna and 
flora are found to be somewhat attenuated when reaching their 
farthest point. Many species in this westward and subsequent north- 
ward and finally eastward push must have lagged behind, and in 
corroboration of this theory we should expect to find conspicuous 
forms which were unable to keep up with the procession and thus 
failed to reach western Norway before the final submergence of 
the land connection. As a matter of fact, there are many such 
species, but it is not necessary to refer here to others than the 
muskox (Ovibos moschatus) and the banded lemming (Dicrostonyx 
torquatus) which are not found in Norway, living or fossil. | 
On the other hand, it must be emphasized that the species and 
forms referred to above are not the only ones which came that 
way, but they are only those of which we know that they differ 
structurally from the other individuals which may have invaded the 
peninsula from other directions. Many plants and animals of gen- 
