ECOLOGY 277 



by the blue gill is close to neutrality while the optimum for sunfishes, 

 members of the same family, is 6 cc. of carbon dioxide per liter. Again 

 certain fishes live in deep water under a pressure which would kill shallow 

 water forms ;" and when these bottom fishes are V^roiight into shallow water 

 or into the air they burst. In this connection it should be pointed out 

 that some animals are apparently adapted to a narrower range of condi- 

 tions than others. Semper is authority for the statement that one species 

 of Neritina (snails) can live equally well in fresh, brackish and sea water, 

 while others occur only in one ( the other and cannot stand any appre- 

 ciable increase or decrease in thv K^inity of the water in which they live; 

 and, again, that the stick\ebsick,^,nr'erosteus aculeatus (a freshwater fish), 

 will live in sea water if supplied )itI h food, while the freshwater clams, 

 Unio and Anodonta, will perish %  rfea water, sometimes within a few 

 hours. 



Adaptation Not Purposeful. — In concluding this discussion of the 

 nature of the relations between animals and the environment it should 

 be noted that in referring to the structural and physiological characters 

 of species which fit them for life under certain conditions as adaptations, 

 it is not understood that the relationship is purposeful. The term is 

 now most generally used to mean the fact of fitness, that is, it does not 

 refer to the cause. Adaptation is advantageous, but not designed. 

 Perhaps the term adjustment would be^ preferable to adaptation. 



- ANIMAL ASSOCIATIONS-' 



In general, as has been pointed out, organisms exist chiefly, or thrive 

 best, in particular conditions. Since different forms may react in a simi- 

 lar way to the same conditions, they are often brought into the same 

 habitat. This is revealed by the presence in each region of animal 

 associations. These associations are the results of interrelations of indi- 

 viduals of different species, of the relations of the individuals of the same 

 species to each other, of the dependence of animals upon plants, and of 

 the relations of the individuals to the physical environment. Some of 

 the relations of animals and the resultant associations are as follows: 



1. Relations between individuals of the same species. 



(a) Colony formation. 

 (h) Societ}^ formation. 



2. Relations between individuals of different species. 



(o) Parasitism. 



(b) Symbiosis. 



(c) Slavery. 



(d) Enemy and prey. 



(e) Mimicry. 



3. Relations of animals to the plant environment. 



(a) Plants depended on for food. 



(b) Plants depended upon for shelter, shade, etc. 



4. Relations between animals and the physical environment. 



