MOVEMENTS OF DAPHNIA 259 



indicates an upward movement of the species. As the individuals 

 grow older they tend to drop into lower levels, and through 

 natural mortality grow few in number. 



We must, then, distinguish a periodicity in vertical distribu- 

 tion due to the aging of individuals and the occurrence of re- 

 productive periods. 



GENERAL FEATURES OF BEHAVIOR IN DAPHNIA 



After a considerable study of these daphnids the conclusion 

 is forced on one that they do not behave in a perfectly mechan- 

 ical manner. They are not little machines which will respond 

 in a definite way each time a definite change is made in the 

 environment. A glance at almost any of the tables included in 

 this paper will make it clear that although there is a tendency 

 to a definite kind of behavior when any particular one of the 

 environmental factors is altered, yet a small proportion of the 

 animals will not react in that particular way, but may even 

 react in a manner directly contrary to the behavior of the 

 majority. It may be that with more exact and thoroughgoing 

 methods of research many of the apparent exceptions could be 

 shown to be due to normal processes. At present the best we 

 can do is to say that environmental changes tend to produce 

 changes in the condition of the animal and that it then reacts 

 differently than before. 



In this paper it has been the aim to show the normal tenden- 

 cies in behavior when a single factor is varied and the others 

 kept constant or eliminated as not contributing to the results. 

 When possible a number of animals were used in the same ex- 

 periment so that quantitative results were secured. Where this 

 was not possible the results have been checked by a number of 

 repetitions of the experiment or by other methods of control. 



In cases where several environmental factors vary at the 

 same time the behavior becomes quite complex and it is difficult 

 to determine in such cases which factor is the more potent and 

 what the resultant behavior will be. 



In the previous part of this paper much use has been made 

 of such terms as phototaxis, geotaxis, etc. It is not intended 

 by the use of such expressions to subscribe to any particular 

 theory as to the basis of animal behavior. By the use of these 

 terms it is intended only to express the tendency to move toward 



