598 ROLEIN DaSALTSBURYV 
if all the migrations could be ascertained. Thus in Scandinavia 
and Denmark, we have a succession of post-glacial floras, the first cor- 
responding in a general way to the present vegetation of the tundra, 
the second a forest vegetation dominated by the birch and poplar, 
the third a forest vegetation dominated by pines, the fourth, one 
dominated by the oak, etc., the fifth a flora similar to that of the 
Black Forest Mountains, indicating a temperature warmer than that 
of today for the same region, and finally, a southward retreat of the 
last flora to its present latitude. The first five members of the succes- 
sion seem to correspond with the half of a normal interglacial series. 
If this interpretation is correct, this series of five floras would be nearly 
doubled with the on-coming of another glacial epoch, and this doubled 
series must have been repeated, substantially, several times in the 
course of the long succession of glacial epochs. Fragments of inter- 
glacial records have been found both in America and Europe. Ina 
few cases they are full enough to encourage the hope that when their 
number is duly increased, they may be pieced together into consistent 
wholes. It is too much to expect that they will ever be as complete 
as the record of post-glacial life. 
It is not now apparent just how far biologic or paleontologic data 
of the Pleistocene, except from their record of climatic changes, are 
to be significant in correlation. Aside from the mammals, changes 
of species have been insignificant. Even among mammals, it is not 
clear that the dying-out of species in one locality was contemporaneous 
with the disappearance of the same species in other localities. A 
stratigraphic basis for this interpretation would be needed before it 
could be accepted. So far as all other forms of life are concerned, 
the paleontologic record of one interglacial epoch must have been 
essentially identical with that of another, if the intervals were equally 
long and mild. 
Perhaps more help in correlation may be looked for in another 
direction. Intercontinental migrations, it would seem, must have 
been virtually restricted to interglacial epochs. The times when 
species first appeared in a given region may therefore prove to be 
much more significant in correlation than the times when species 
died out. 
Something perhaps may be hoped for in the careful study of the 
