52 
ORIGIN OF LIFE IN AMERICA 
the destruction of the species in the drainage area of the 
Upper St. Lawrence. What other cause but the Glacial 
Epoch can be assigned for this partial destruction ? And 
in explanation of this and other similar cases it seems 
reasonable to conclude that certain areas within the drift 
region were entirely free from the devastating agency, be it 
land-ice or the sea, which destroyed so much of the pre¬ 
existing fauna and flora. We might- argue, of course, that the 
pearl-mussel had advanced northward in two independent 
streams in post-Glacial times, one along the Mississippi valley 
and the other along the Atlantic coast, and that they had thus 
reached the northern habitats alluded to. But quite apart 
from the difficulties connected with pushing its way along 
the coast from river to river, there seem to me to be no 
grounds for such a supposition. The fact that Margaritana 
margaritifera is the only fresh-water mussel (Naiad) belong¬ 
ing to the Atlantic river system which is now found living 
within the Mississippi drainage area, shows how inconceiv¬ 
ably slowly these mollusks spread. The fresh-water mussel 
fauna, so characteristic of the Mississippi drainage area, is 
believed to have descended in a scarcely broken line from that 
of the Cretaceous Period, and many recent forms are only 
distinguishable with difficulty from those that lived in those 
remote times. 
However, the question of survival or non-survival in the 
drift area is not confined to the instance alluded to. Lampsilis 
(Unio) borealis, Lampsilis superiorensis, Anodonta mar- 
ginata, Anodonta kennicotti, and Anodonta pepiniana are all 
restricted in their range to the drift area, and are not known 
to have ever lived outside it.* 
There is no doubt ample zoological evidence for the assump¬ 
tion that many species of vertebrates and invertebrates 
survived the Glacial Epoch within the area commonly sup¬ 
posed to have been covered by vast ice-masses. But there 
is another important point which the geographical distribu¬ 
tion of the fresh-water pearly mussels may elucidate. Several 
times I have referred to the theory that in pre-Glacial times 
the land between northern Labrador and eastern Massa- 
* Simpson, C. T., “ Synopsis of pearly freshwater mussels.” 
