140 



ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



they depart from the optimum, and thus have a selective action. Forms 

 that cannot survive extremes must perish (cf. Chapter XI). In regions 

 of optimum physical environment, such as tropical seas and the tropical 

 rain-forest, the biocoenoses are characterized by numerous negative 

 features, by the absence of those limitations which the adverse con- 

 ditions of less-favored regions place upon their faunae. Any permanent 

 change in the physical environment (as in the biotic) conditions a 

 change in the biocoenosis. Some members will drop out, others will 



§ § 







70000 

 60000 

 50000 

 40000 

 30000 

 20000 

 10000 

 



Fig. 11. — Receipts of skins of Lepus americanus (above) and Lynx canadensis 

 (below) by the Hudson Bay Company from 1845 to 1905. After Seton. 



thrive better, still others will be enabled to enter. The lakes of Finland 

 and south Sweden were once part of the Littorian Sea; their separation 

 was followed by freshening, and the entire marine fauna was reduced 

 to a single fish (Cottus quadricornis) and a number of crustaceans 

 (My sis relicta, Idothea entomon, Pontoporeia affinis, and Limnocalanus 

 macrurus) . Similarly, climatic changes at the close of the glacial period 

 have completely changed the mid-European biocoenoses. 



On account of this selective influence of the environment upon 

 animal life, the ecologic communities in widely separated parts of the 

 world may be similar, if similar processes and conditions are con- 

 cerned. Thus corresponding biotopes with similar biocoenoses arise in 

 different parts of the biosphere, quite independent of each other. They 

 are, however, analogous formations. The members of the biocoenoses, 

 the families, genera, and species, are often entirely different. The most 



