STUDIES IN ANIMAL AGGREGATIONS. 315 



physiologists, warrants the postulation of another and more subtle 

 integrating factor in animal communities ; and, when the observed 

 effects of pond mosses are remembered, in biotic communities 

 also. This integrating factor might well be called the auto-pro- 

 tective value of the community. Further investigations concern- 

 ing the extent of the phenomenon in nature are needed, but the 

 indications are that the secretion of such substances as slime, and 

 probably even the accumulation in the surrounding environment 

 of a certain concentration of what are ordinarily called waste prod- 

 ucts, help render a raw environment more suitable for many 

 forms. 



Drzewina and Bohn found tadpoles resisting colloidal silver 

 more readily if the solution were made up in pond water rather 

 than in tap water. They interpreted this as being due to the 

 presence of some dilute auto-protective tadpole secretion, but 

 from our experience it seems more probable that the protection 

 was due to the presence of slime or other material secreted by all 

 pond animals. 



It is a commonplace of ecology that many species of plants and 

 animals so react upon their environment as to limit the continued 

 existence of their species in that locality, while at the same time 

 preparing the way for their successors. This general condition 

 is well illustrated by the work of Woodruff on the sequence of 

 protozoan cultures. We are proposing an extension of this prin- 

 ciple to include the idea that the pioneers in a sucession first react 

 upon their environment so as to render it more favorable for sup- 

 porting themselves and their own associates. It seems probable 

 that a continuation of the same processes and further accumulation 

 of the same products that at first are favorable will ultimately 

 become unfavorable and so help cause ecological succession. 



Conditioning of a raw environment to support a population is 

 not limited to accumulation of favorable chemicals, for recent work 

 (Alice, 192/a) indicates that in the absence of customary physical 

 elements, as of eel grass in the case of Ophiodcrma, the animals 

 may so react with each other as to substitute their own bodies for 

 the missing grass. 



It must be admitted that to date, most of the data supporting 

 this view depends on laboratory studies made largely upon reagents 



