58 MICHIGAN SURVEY, 1908. 



ture or rainfall may certainly keep pace with the changes in lake level, 

 or even be caused by it, and in either case they would exert a profound, 

 influence on the biota. Migration of species is still taking place among 

 both plants and animals, and may introduce new or even dominating 

 species ani,ong the present forms. The so-called equatorial pressure of 

 southern species is fully as s-trong now as it was directly after the close 

 of the glacial epoch. Lastly, and most important of all, the influence 

 of the biota itself is always to be reckoned with. Both plants and 

 animals are continually becoming more plastic, adapting themselves to 

 new conditions, and extending their habitats into new associations. 

 They push forward more rapidly than the changes in physiography, some- 

 times hastening and sometimes retarding physiographical action, and 

 at all times greatly influencing the subsequent successions. 



A biotic association may develop into another by a mere re-arrange- 

 ment of the interrelations, numerical or otherwise, of the component 

 species, without the necessary loss of some or addition of others. But 

 such cases are rare, and the Isle Eoyale observations show that no two 

 associations have exactly the same species, and that with each pro- 

 gression there has been an addition of certain forms which become the 

 most characteristic types. The first bit of beach formed was occupied by 

 an association possibly not unlike that of the present beaches. All the 

 species must evidently have immigrated from beyond the island. When 

 the soil deposits on the beach were sufficient to support a second as- 

 sociation its species were derived partly from the beach itself and partly 

 from new immigrants. The further development of biotic associations 

 on the beach was then possible not only from immigrants, but also from 

 the two associations already present. Similarly at the present time 

 each association on the island is constantly being invaded by species 

 from all the others, and many of them are actually able to establish 

 themselves. This tends toward a homogeneity in the biota hardly in 

 full accordance with the recapitulation idea. Indeed, it is very probable 

 that independently of all physiographic agencies the whole surface of 

 the island would eventually be occupied by the balsam-spruce forest and 

 its attendant fa unal association. 



In many cases it is virtually certain that the lateral succession does 

 faithfully repeat the vertical, and the zonation of plants around a pond 

 may be taken as an example, but the filling of a pond is only a single 

 step in the genetic development of the biota of an island. 



With this preliminary note of warning, the truth of this recapitulation 

 theory will be assumed for the island, and the discussion of the insect 

 and molluscan fauna will follow the genetic lines indicated in the fii*st 

 paragraph. 



The relationship of the various physiographic types on the island 

 to each other may conveniently be expressed by a diagram, (see end of 

 paper), indicating the direction of the devolpment by arrows. It must 

 be reiuembered that pra.ctically any one of the intermediate stages may 

 be omitted. 



II. The Lake. 



The lake (Superior) must obviously be regarded as the first stage in 

 the genetic development of the faunal associations. Broadly speaking, 

 the lake fauna is divisible into two main groups. The first is pelagic in 



