54 OEIGIN OF LIFE IN AMEEICA 



nearly obliterated. This change in form is so striding, 

 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 horn 

 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 repla.ced 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 me 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. 



