

22 THE CHICAGO ACADEMY OF SCIENCES. 
the steady pull of the rapidly flowing water. Examples of Lymnzas 
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 :* 
“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 high 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 Lymneas 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.* 
There is an interesting habitat at the southern end of Cayuga 
Lake, New York, in which a clear brook flows through the center 
of aswamp. 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: 
‘Trans. Acad. Sci., St. Louis, XVI, p. 2. 
“Ecological survey of Michigan, 1906, pp. 28, 24. 
