4 JEAN DAWSON 



The first named object involves the following steps: 



(a) A study of the gross environment (lake and river systems) 



to determine the general conditions under which Physa 

 lives. 



(b) An analysis of the conditions prevailing in each locality 



occupied by the snails in order to determine the ele- 

 ments making up the habitats. 



(c) A comparative study of the habitats and the determination 



of the most favorable conditions for snails. 



It is evident that in comparing one habitat with another, 

 there must be some standard of comparison. I have there- 

 fore taken in each bod}^ of water the habitat that yields the 

 greatest number of snails, and have called this the "optimum" 

 for that water, and have compared it with the other habitats 

 having a less number of snails. In general the relative suit- 

 ability of a snail habitat may be estimated by the number found 

 flourishing in it. In a body of water where the agents of dis- 

 persal are constantly at work, one can have but little doubt 

 that the snail and its eggs will be thoroughly dispersed through- 

 out its extent. Whene^'er snails occur in very favorable con- 

 ditions they multiply and flourish in great numbers. Less 

 favorable conditions produce of necessity a fewer number of 

 snails; and in some places these conditions are so unfavorable 

 that the snails perish, and thus fail to propagate their race. 

 It is largely due to the fact that a single body of water offers 

 such a variety of conditions, that we usually find a discon- 

 tinuous distribution of snail life in it. 



The second object may be more fully indicated as follows: 

 After once establishing the elements making up a Physa hab- 

 itat, it is desirable to study the causes which are continually 

 modifying these conditions and producing changes which make 

 for or against the optimum conditions for the snail. These 

 changes are caused by both natural and artificial means. The 

 natural changes affecting the given elements in a stated place 

 may be w^ll predicted \\'ithin a certain limit, but the artificial 

 ones are more difficult to anticipate. 



III. The Brook-Creek-River System as a Habitat. • 



The relation which the snail bears to the river system may 

 be better understood , perhaps, if the river habitats are studied 



