ECOLOGY OF ISLE ROYALE. 35 



The time of invasion and the sources of the Glacial and post-Glacial 

 supply of life which invaded the northern land and waters presents 

 several interesting problems. At this point the origin of the aquatic 

 biota is of special interest. From what is known of the fresh-water 

 aquatic biota of the far north, it is very probable that the Glacial Great 

 Lakes, at least in their later stages, were not utterly devoid of animal 

 life. So it seems reasonable to infer that ^uch forms as lived during 

 Glacial times near the ice margin were among the first to succeed the 

 retreating ice sheet. 



As the lake fauna is one of the most characteristic features of the 

 life of northeastern North America, the question naturally arises as to 

 where this fauna was preserved during the Ice Age. Today there is 

 no extensive development of lake fauna south of the glaciated region. 

 Where then was such a fauna preserved? On the west were the arid 

 plains, and to the south an old land area of mature drainage and very 

 few lakes. Such relations as these suggest that this fauna must have 

 occupied the lakes formed on the rejuvenated glacial topography or 

 in the adjacent streams. Had there been extensive lake areas to the south 

 to act as regions of preservation, it seems probable that the present fauna 

 of the Great Lakes would have been much richer. Undoubtedly 

 the most important fresh-water preserve was the Mississippi system, 

 on account of its direct communication with the glacial drainage, thus 

 allowing a southward escape into more favorable climatic conditions. 

 This was also a water communication of considerable duration, lasting 

 even into early post-Glacial times, and one which has greatly influenced 

 the origin of the present fauna of the Great Lakes. In all probability 

 it was this Glacial and post-Glacial water connection and barrier 

 that retarded the northern extension of so many land species, and at the 

 same time favored the extension of certain aquatic animals. The later de- 

 velopment of the eastern outlets did not open up such a favorable 

 source of supply as occurred farther west. 



So much for this phase of the problem. Now let us continue with 

 the history of the Superior basin since Algonquin times. The fall in 

 the lake level did not take place suddenly, since a series of beach 

 lines are preserved which show that it halted for some time at different 

 levels, but none were of any remarkable duration until it reached a 

 level marked by a very extensive series of beach lines now preserved 

 at about 60 feet above the level of Lake Superior. This well defined 

 beach represents the shore of the Nipissing Great Lakes, post-Glacial 

 lakes whose general outline was much like that of the present lakes 

 in the same basins, as is indicated in Fig. 54- The low outlet of this 

 lake was to the east through the Ottawa valley into the Champlain 

 Sea, and is of special interest in that it is suggestive of how certain 

 Great Lake animals of marine affinities {Mysis, Pontoporeia, Triglopsis) 

 might have invaded the upper lakes in post-Glacial times. At one 

 time it was thought that there had been a Glacial salt water com- 

 munication between Lake Superior and the Hudson Bay region, but 

 this view has been abandoned (cf. Taylor, '97, pp. .127-128; '96, pp. 255- 

 256, and Coleman, '06, pp. 19.3, 198-199). It is definitely known that 

 the land was depressed to the north of Lake Superior, but this period 

 of depression was at a time when it was covered by the ice sheet, 



