22 THE CHICAGO ACADEMY OF SCIENCES. 



the steady pull of the rapidly flowing water. Examples of Lymnaeas 

 living in this kind of a habitat are Galba apicina, in Union River, 

 Michigan, and Galba reflexa in a rapidly flowing stream near Lockport, 

 Illinois. 



That the ecological study of nature is of great value in the sepa- 

 ration of species and races has been clearly demonstrated by the ex- 

 haustive examination of local areas, which has shown that the variation 

 of the individual is in direct ratio to the variation of the environment. 

 A case in point is recorded by Dr. W. A. Nason, who thus describes 

 the molluscan habitats near Alpena, Michigan: 1 



"Thunder Bay Island is one mile east of the north point of 

 Thunder Bay. Here I found the shells in pools of water in ledges 

 of limestone which are accessible to the waves of the lake during 

 storms or h'gh water. Sugar Island is a low island between Thunder 

 Bay Island and the mainland, a little to the north. 



"Sulphur Island is a small, low island near the southern point 

 of Thunder Bay, about eight miles from the city of Alpena. Long 

 Lake is about twelve or fifteen miles north of Alpena, and its shores 

 are mainly limestone rock and low bluffs, except at the outlet of the 

 creek which flows into Thunder Bay River. The shells were mainly 

 collected on a broad, sandy beach about the outlet, and in from six 

 inches to a foot of water. 



"The change of localities gave the shells a difference in develop- 

 ment which is very interesting. The contrast between the solid forms 

 of Thunder Bay Island, where the waves and cool water of Lake Huron 

 was almost daily forced into the pools where the shells developed, 

 and the still, swampy pool where the Sugar Island Lymnseas lived, 

 where the shores were low, but protected from the rough storms of 

 the lake, give a hint of conditions very interesting, biologically." 



Mr. A. G. Ruthven describes a somewhat similar habitat which 

 occurs on the shore of Lake Superior, Ontonagon County, Michigan, 

 inhabited by Galba apicina. 2 



There is an interesting habitat at the southern end of Cayuga 

 Lake, New York, in which a clear brook flows through the center 

 of a swamp. The still, stagnant pools are inhabited by Galba palustris, 

 to which location they are confined. The running stream is tenanted 

 only by Physa and Galba umbilicata, the latter being scattered over 

 the surface of the mud bordering the stream. 



Mr. Junius Henderson, of the University of Colorado, thus de- 

 scribes the ecological features of Colorado: 



. Acarl. Sci., St. Louis, XVI, p. 2. 

 2 Ecolog'ical survey of Michigan, 1906, pp. 23, 24. 



