52 OEIGIN OF LIFE IN AMEEICA 



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 faupa 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 Margaritaoia 

 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 moUusks 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 instaiice alluded to. Lampsilis 

 (Unio) borealis, Lampsilis superlorensis, 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 assum!p- 

 tion that many species of vertebrates and invertebrates 

 survived the Glacial Epoch within the area commonly suj)- 

 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 Ma;Sea- 



■* Simpson, C. T., " Synopsis of pearly freshwater mussels." 



