BIOLOGY OF PHYSA 3 



the animal is found in one part of a given body of water rather 

 than in another. 



Preference for a definite habitat is one of the most important 

 phenomena in the local distribution of an animal. This definite 

 tendency was continually met with in the field work on Physa. 

 It is well illustrated by the occurrence of the snail in a certain 

 tamarack swamp whose invertebrate life was studied in detail. 

 This swamp, which lies south of Kavanaugh Lake in Wash- 

 tenaw county, Mich., is a lake basin, formerly of greater extent, 

 and is surrounded by morainic hills. Only a relatively small 

 portion of its 300 acres is covered with tamarack trees. There 

 are two small ponds in one section of the swamp, which have 

 several streamlets and one large ditch draining into them. A 

 large ditch drains both, as there is still a slight connection 

 between them. 



Numerous ditches and streamlets occur in the swamp and 

 serve to lower the water level so that standing water is found 

 only in the deeper holes and small pockets, which occur in less 

 than one-half of the entire swamp area. This swamp is a good 

 locality for the study of local distribution because it has many 

 conditions common throughout its extent. It has a surface 

 substratum of peat, and since flooding took place during spring 

 freshets before the ditches were dug and also since that time, 

 the question of opportunity for dispersal may be eliminated. 

 If Physa is not found in certain spots in the swamp where there 

 is water the year round, it is probable that this is not because 

 it was unable to reach these places. The accompanying dia- 

 gram. Fig. I, shows the standing water in the swamp. The 

 places where Physa is found are dotted and its relative abun- 

 dance is shown by the number of dots. 



A glance at the diagram will show that Physa is abundant 

 in some waters and absent in others. The question therefore is. 

 Why this kind of distribution and what conditions determine it ? 



Before giving in detail the observations made upon Physa in 

 the different bodies of water, I wish to set forth briefly the main 

 objects and methods of the work. They are two, as follows: 



1. To determine the combination of conditions which make 



the normal abode of Physa. 



2. After obtaining these combinations, to determine the 



causes which modify them. 



