CONDITIONS OF EXISTENCE 55 



fall dilutes the plankton and in "streams carries it away. Too 

 little sunshine causes a poor production of the chlorophyll bearing 

 organisms which are a food basis of others. Open winters favor 

 denitrification and may be unfavorable to certain lower invertebrates. 



Animals fed upon certain kinds of food supply enzymes digest- 

 ing that kind of food in the proper quantity. The proportion of 

 the different kinds of enzymes changes with changes in diet. Under 

 proper experimental conditions anti-pepsin, an ti- trypsin, etc., are 

 developed by organisms. Organisms may develop immunity to 

 toxins introduced into the alimentary canal with food, but the 

 process is a slow one. The introduction of toxins, or bacteria re- 

 producing them, directly into the blood is doubtless a common 

 thing among aquatic animals which are probably as subject to 

 injury and disease as are land animals (see Hill or Rosenau). 

 Various aquatic organisms must possess natural immunity for the 

 various decomposition products of fresh water (see under bacteria, 

 p. 94). Acclimatization must often involve the development of 

 immunity. As knowledge along these lines is increased the con- 

 viction that enzymes, toxins, immunity and related phenomena 

 play a very important role in the life of fresh-water animals grows 

 proportionately. Lillie has recently found that comparable phe- 

 nomena are of great significance in connection with the fertiliza- 

 tion of the eggs of marine animals and future investigation along 

 these lines will doubtless be of much importance. 



Ecological classification must be based upon community of phy- 

 siological make up, behavior, and mode of life and similarity of 

 habitat. Those natural groups of animals which possess likenesses 

 are the communities which must be recognized. One community 

 ends and another begins where a general more or less striking 

 difference in the larger physiological characters of the organisms 

 concerned occurs. These communities generally occupy relatively 

 uniform environments. For any given organisms the other organ- 

 isms of the community are a part of the conditions of existence 

 There is general agreement in the recognition of strata, of associa- 

 tions as communities based upon minor differences in habitats, 

 and formations based upon larger major differences in habitats 

 and considerable agreement in the use of consocies and mores. 



