54 
ORIGIN OF LIFE IN AMERICA 
nearly obliterated. This change in form is so striking, 
says Dr. Simpson, that many of these northern races of 
Mississippi mussels have been described as distinct species. 
Dr. Simpson is of opinion that this remarkable change in 
form among the Mississippi species inhabiting the St. Law¬ 
rence drainage system has been produced since the Ice Age 
began to draw to a close, because, he argues, it is almost 
certain that all fluviatile and lacustrine life under the ice 
sheet was destroyed. 
The prevalent geological opinions as to the nature of the 
Ice Age thus dominate all biological thought, in reference to 
problems of distribution. If we emancipate ourselves from 
these pre-conceived notions in our speculations on the origin 
of the existing fresh-water mussel fauna, we must arrive at 
different conclusions. 
Two almost diametrically opposed theories have been pro¬ 
pounded to explain the origin of the geographical features 
of the river system of north-eastern North America in pre- 
Glacial times. According to Professor Grabau,* Mr. Warren 
Upham and Professor I. C. Russell are of opinion that the 
greater part of the Laurentian basin formerly discharged its 
waters southward to the Mississippi. With this view Professor 
Grabau agrees. Much of the Archaean region in north-eastern 
North America must have stood greatly higher than at present, 
the slope of the land being toward the south-west. Two great 
rivers then existed, according to Professor Grabau, in place 
of the present Great Lakes, the Saginaw and the Dundas 
Rivers, both of them draining into the ancient Mississippi. 
Dr. Spencer is in agreement with Dr. Grabau in so far as he 
contends that the land stood at least two thousand feet higher 
than at present. He differs from the other writers in his 
belief that the lake region and the more north-easterly high¬ 
lands were equally affected by the uplift, so that the rivers 
which replaced the lakes in pre-Glacial times drained into 
the St. Lawrence just as the waters from the lakes do now.f 
The first of these two theories seems to mo to explain the 
origin of the eastern fauna in a satisfactory manner. The 
* Grabau, A. W., “ Geology of Niagara Falls,” pp. 42 — 54. 
T Spencer, J. W., “Falls of Niagara,” pp. 400—405, 
