314 W. C. ALLEE AND J. F. SCHUETT. 



by postulating the production of an auto-destructive secretion. 

 Their most impressive test for the presence of such an auto-de- 

 structive secretion was similar to that for the opposite effect, 

 viz., the placing of a single individual into a solution in which 

 a group of animals had died, and comparing the survival time 

 of such an animal with one isolated into freshly prepared solu- 

 tion. We have no evidence to present on this point at present, 

 but before accepting the hypothesis of the production of some 

 specific auto-destructive secretion, we should need to examine 

 the extent of the phenomenon, and the type of reagents that 

 cause a marked increase in the destructiveness of the mass, par- 

 ticularly with regard to their effect on oxygen consumption and 

 the elimination of waste products. 



The deleterious effect of the crowding of many animals into 

 close quarters has long been known and may be easily demon- 

 strated. Goetsch has recently shown that different factors pro- 

 duce the unfavorable effect with different animals ; thus, with 

 Hydra, decreased food supply is most important; with Planaria, 

 equally fed, the accumulation of waste products becomes the limit- 

 ing factor ; with tadpoles, the deleterious effect appears most 

 closely correlated with frequent contacts between different mem- 

 bers of the mass. Limitation of available oxygen has the same 

 effect with all. Until it is demonstrated that chemicals such as 

 KC1 cannot produce the observed increase in the menace of the 

 mass by their known effects upon one or all of these relations, it 

 is idle to speculate concerning hypothetical auto-destructive se- 

 cretions. 



Our experience that protection is furnished by heterotypic ag- 

 gregations as well as by those of the same species has important 

 ecological implications. Hitherto biotic communities have been 

 regarded as being organized by similar or dovetailing requirements 

 which the organisms make of their environment. These may lead 

 them to dwell in the same or in similar habitats, while animals 

 with other requirements separate into a different community. Com- 

 munities are also considered to be integrated by their food inter- 

 relations. (Forbes, Shelf ord.) 



The work with colloidal silver, here reported, taken together 

 with the observations of Bresslau and the experiences of the plant 



