ECOLOGY OF ISLE ROY ALE. 47 



at the time the center of the storm is passing." To this class belong 

 the majority of lake storms, but they are seldom severe. 



From these relations it is seen that storms whose origin is from the 

 south, southwest or mid die- west, are preceded by east or northeast winds 

 or (middle-west) by southern winds, and followed, after the passage 

 of the storm center, by northwest or west winds ; while storms of north- 

 western origin are preceded by south or southwest, and followed by 

 west and northwest, winds. These facts show that offshore winds from 

 the eastern and southern shores of Lake Superior are. the general law 

 for winds prieceding most storms; and that after the passage of the 

 storm center all appear to be followed by west or northwest winds. These 

 offshore winds are likely to be onshore winds for Isle Eoyale. The 

 proximity of the north shore, the frequency and magnitude of this wind 

 phenomena, clearly suggesjts that these factors may largely account for 

 the Canadian aflSnities of the majority of the Isle Royale biota. But we 

 shall see later that there are other factors to reinforce this same ten- 

 dency. It may seem unnecessary to enter these details, but it should b© 

 ■ remembered that the conditions under which an organism may reach the 

 island is an important factor in its survival, a relation of special import- 

 ance in the migration of birds. That the majority of these storms occur 

 in the fall and winter, sit a period of relative inactivity on the part of the 

 Isle Koyale biota, is yet a condition which would be favorable for the 

 transportation of some small hibernating invertebrates. The life histories 

 of these storms, especially the oonditions of their termination, may be 

 expected to have an important bearing upon the survival of the drift 

 biota. 



There is still another important phase of this subject, and that is 

 the influence which these storms have upon the life of the shore and 

 beaches. The fauna of the exposed shore of Isle Royale is very scanty 

 and much inferior to that of the harbors, so that, generally speaking, up 

 to a certain point the more protected the coast the more diversified the 

 fauna. This was very clearly shown by the molluscan life upon the 

 shore. These storms have ia powerful scouring action with the sand, 

 gravel and shingle on the exposed coasts, so that a rock surface or one 

 with blocks too large for disturbance by the waves is much more favor- 

 able to life. 



The relation of waves to lake currents presents a significant phase 

 closely related not only to the occurrence and distribution of life along 

 the beach, but also to the problem of lake drift and its biological im- 

 portance. A breaking wave tends to carry forward floating objects so 

 that when such objects are carried along by the currents and once come 

 within the range of influence of the breaking waves of shallow water, 

 they tend to move with these waves into the shallow water and thus 

 shoreward and are cast upon the beach in harbors, bays or about 

 islands (Harrington, '95, p. VI.). 



e. The Surface Currents of Lalce Superior. Mention has previously 

 been made of the fact that in addition to the offshore winds from Canada, 

 which accompany certain severe storms, there are other influences 

 which have a similar eft'ect upon drift — the lake currents. These are, 

 in part, an expression of the same climatic trend and their direction 

 is a resultant determined by the influence of the prevailing westerly 



