ECOLOGY OF ISLE ROYALE. 37 



and thus the salt water was excluded. An alternative hypo'thesig Is 

 that these animals are adapted to a constant and low temperature 

 rather than to fresh or salt water, and that during Glacial times they 

 were dispersed far to the south in fresh Avater and have only been 

 preserved in restricted favorable localities. The low temperature of 

 Glacial times would be a period especially favorable for the acclimatiza- 

 tion of marine forms to fresh water on account of the favorable con- 

 ditions which accompany the slow rate of changes at low temperatures. 

 The long duration of the Nipissing Great Lakes is well attested by 

 the character of the beach. As Taylor ('96, p. 398) remarks: "It is 

 altogether the most remarkable littoral feature of the Great Lake 

 region. It is a shore line well advanced towards old age. All other 



beaches of the lakes are youthful in comparison Instead ' 



of the slender spits and barrier bars of the Algonquin and other beaches, 

 the Nipissing beach has what may be called barrier plains, made up 

 of many, sometimes forty or fifty, massive beach ridges laid one against 

 the other. Many bays were entirely filled by these beach plains and 

 others were cut off, so as to form small littoral lakes. Some of these 

 plains are a mile to a mile and a half wide. In some instances the old 

 deltas of other beaches are large and conspicuous, but the constructive 

 products of wave action have no comparison to those of the Nipissing 

 beach." From a biological standpoint these facts are of special signifi- 

 cance. The maturity of the beach line is a condition decidedly favor- 

 able to the development of a littoral biota. The sandy shore, spits, 

 bars, beach pools, cut-off ponds and lakes furnish a variety of favorable 

 habitats in marked contrast with the poverty stricken character of 

 life frequenting an exposed and topographically youthful lake shore. 

 Such an old beach is both qualitatively and quantitatively favorable 

 to the biota, and not only favors an abundant supply but also its dis- 

 persion along shore and by currents throughout such a body of water. 

 The long duration of such conditions is of evident advantage to an 

 extensive dispersal of such life. 



As the basin of the Nipissing Great Lakes in the Superior basin was 

 so much like that of Lake Superior, it is not improbable that the lake 

 currents were much the same in both lakes, so that our knowledge of 

 the present lake currents should aid in the interpretation of those of 

 the Nipissing Great Lakes. Such relations as these suggest that at the 

 Nipissing stage, and perhaps even earlier, the lake currents tended to 

 people Isle Eoyale with north shore drift. By this time the island 

 was quite large, though smaller than the present island by the subtraction 

 of the area; below the 60-foot contour. At this time the climate of the 

 region must have become greatly ameliorated so • that the north shore 

 of Lake Superior was perhaps repopulated tvom the south, largely 

 around the western end of the lake. ^ With the advent of an abundance 

 and diversity of plant and animal life, a new element enters the environ- 

 ment, whose influence is far reaching. The vegetation tends to blanket 

 the surface with a humus layer and thus to bind the soil so that it 

 retards erosion and becomes a geological agent. The influence of 

 animal life is also far reaching and may be conspicuous if beavers are 

 abundant. But these influences will only be mentioned here. 

 The development of the Nipissing beach upon Isle Eoyale has not 



