ECOLOGY OF ISLE ROY ALE. 59 



character and includes those species wliose distribution is entirely inde- 

 pendent of the shore, for example, most of the species of fish. The 

 second group is littoral; the species occur along the shore in compara- 

 tively shallow water, and are to a greater or less extent dependent upon 

 the land in its relation to the character and slope of the bottom and to 

 the motion of the water. Members of the latter group only are con- 

 sidered here. 



The two dynamic factors just mentioned are the most important ones 

 that influence the biot.a of the lake. There are no currents of sufficient 

 rapidity to affect the animal life. The direction of the wind, whether 

 oflf-shore or on-shore, may respectively lower or raise the level a few 

 centimeters, especially when the wind blows lengthwise of the long 

 narrow inlets, such as Conglomerate Bay {Fig. 11). Some fixed or 

 slow-moving species may accordingly be alternately submerged and ex- 

 posed, while motile forms can at once adjust themselves to any change 

 of level. Of far greater importance is the motion of the water caused 

 by wave action. It is only on rare occasions that the lake is quiet. 

 Gentle waves come in nearly all the time, and after storms become of 

 great violence. Wave action is of itself suflBcient to inhibit the growth 

 of shells along the exposed shores, where they might easily be torn 

 loose and crushed against the rocks. Such forms are consequently re- 

 stricted to the shores of the smaller bays or to the lee side of islands. 



Wave action is of importance further in determining the character 

 of the bottom. Where the shore is exposed directly to the lake it is 

 usually of massive rock, all the fragments having been washed down to 

 deep water. In small shallow coves, where the waves break always in 

 one direction there is usually a sloping beach of gravel extending across 

 the end perpendicular to the direction of the waves. Every breaker 

 sorts over this gravel so that it is nearly impossible for a fauna to 

 develop. In larger coves or bays, where the violence of the wave action 

 is reduced by distance, the gravel is finer or even a beach of sand may 

 rarely be formed. Along the steep or clifE-like sides of these coves the 

 bottom is frequently covered with angular rock fragments too large 

 to be moved by the water. These are frequently inhabited by shells. In 

 general the development of a free littoral fauna demands quiet water 

 where the animals will not be dashed on the rocks or stranded on the 

 shore, and for attached ispecies there is required either quiet water or 

 a firm bottom which will not be dislodged by the waves. A more de- 

 tailed discussion of this as afl'ecting the distribution of shells will be 

 given later. 



In the larger inland lakes, of which Siskowit Lake, the only one of 

 the class studied, may be taken as an example, essentially the- same con- 

 ditions obtain as on Lake Superior itself. The difference in temperature 

 and content of the water seems to be of minor importance. The waves 

 in the larger lake can naturally reach a larger size, and their influence 

 is felt far into the bays. Thus at the head of Rock Harbor, about six 

 kilometers from the lake proper, the distribution of shells and the al- 

 most total absence of free forms indicate that even there wave action 

 is of importance. In Siskowit Lake, although larger than Rock Harbor 

 the force of the waves is so reduced by every headland or island that 

 on the quiet water in their shelter a rich fauna of such free forms as 



