392 MICHIGAN SURVEY, 1908. 



marginal zone upon the open or deeper water. At the same time this 

 marginal open zone, as a solid substratum develops, tends to become in- 

 vaded by Tamarack, Black Spruce and Arbor Vitae, and still later by the 

 balsam and white spruce forest. But while attention has only been 

 directed to the conspicuous forest cover, it should be remembered that 

 tlie entire environment, the water, soil, ground cover, light relations, 

 animal foods, etc., are also undergoing a transformation. 



Correlated with the invasion of the open swamp by the forest is the ar- 

 -rival of the Eed Squirrel ; while as the forest becomes denser and a shade 

 develops under the trees conditions are produced which are favorable 

 fo]- the Red-backed Mouse. These forested swamps are likely to have 

 a poor ground fauna, as the forms likely to frequent the open are 

 greatly reduced in numbers or excluded, while the wet ground tends 

 to exclude many forms of the balsam forest. But as these forested 

 swamps become dryer, the balsam and white spruce tend to invade 

 them and thus one is able to see all stages of transition, from the open 

 water to that of the balsam-spruce forest. With regard to the mammal 

 fauna, these relations may be briefly summed up as follows : from the 

 open water to the balsam-spruce forest there is a relatively simple 

 change, from the dominance of the aquatic and marsh types (supple- 

 mented by the bats) to land forms which are terrestrial, as the weasels, 

 terrestrial and arboreal, as the Lynx, and arboreal as the Marten, and 

 aerial as the bats which frequent the margins. 



Let us now consider the second series, which begins with land rather 

 thnn open water, and trace its general succession. 



2. The Land, Series. — As the lake level fell from the island, rock 

 surfaces were exposed which surrounded the wet and damp depres- 

 sions. In all probability these surfaces had but little soil, like the 

 exposed wave-washed beaches of today. These flat rock surfaces and 

 ridges have probably had quite different histories or successions from 

 that of the depressions, although both were originally open, yet this 

 was due to very different causes; in the case of the lake this may have 

 been because a substratum was lacking, while on the rock surface there 

 was no soil and hence the openings or "rock clearings." Thus bare 

 or lichen covered rocks offer little that is attractive to mammals, 

 although bats might take shelter here during the day under loose 

 rocks, and partrol the open at night; yet it is not until there has been 

 an accumulation of soil in the crevices, so that the Bearberry, Pennsyl- 

 vania Cherry, Cladonia or scattered Jack Pines get a foothold, that the 

 Varying Hare, Red Squirrel and Caribou can find their food here. In 

 turn comes the Lynx, Weasels and perhaps the Marten in search of 

 the vegetarians. Here again the Bats, Red Squirrel, Hare and 

 Lynx are pioneer mammals invading open unforested areas. As the 

 soil increases in depth on such surfaces, a bordering zone of Aspen 

 and Birch spreads over the surfaces and slopes in a manner similar 

 to the encroachment of the sedge zone upon the open water of a lake, 

 and tends to restrict the open areas. These in turn are followed by a 

 zone of Balsam and White Spruce, so that in time these surfaces tend 

 to become completely forested, juist as the depressions tend to have a 

 similar fate. With these forests comes the exclusion of the bats, while 

 il'e Red Squirrels increase, and the Hare tends to frequent the forest 



