ZOOGEOGRAPHY 309 



covering the enclosed area with symbols or shading. At present an in- 

 creasing number of museums are endeavoring to preserve ecological data, 

 and in time if should be possible to describe the range in terms of the en- 

 vironmental conditions; for example Stejneger and Barbour describe the 

 range of the small blind snake Siagonodon huniilis as "Deserts of Arizona, 

 southern California, Lower California and northwestern Mexico." An 

 advantage of this method is that it permits the student to judge of the 

 possible completeness of the data, for if the principal range of a form as 

 indicated by the records lies within a particular environment it is more 

 liable to occur in parts of this environment which have not been explored 

 than in adjacent areas having different conditions. 



Many of the generahzations in zoogeography have been concerned 

 with attempts to discover and dehmit so-called natural regions. The 

 fact that several species of animals are apparentl}'- limited in range by 

 one factor in the environment, such as particular temperatures, and that 

 conspicuous species of plants may have the same limitations of range has 

 led students to attempt the delineation of areas on the basis of certain 

 features and aggregations of forms. Thus the world has been divided 

 into major regions or realms and the contments into smaller provinces or 

 life zones. A critical study of these regions shows that they are princi- 

 pally based upon one or a few groups, for example, mammals, or mammals 

 and birds; that students of different groups fail to agree on the extent of 

 the regions; and that, as far as the inhabitants are concerned, they are 

 based upon averages. In other words, neither different groups nor dif- 

 ferent forms in the same group respond equally to any one feature in the 

 environment. This is what we are led to expect from the data of ecology. 

 It would seem to follow that since each kind of animal is adapted to cer- 

 tain specific conditions, though there may be certain general regions for 

 particular groups, the search for faunal regions applicable to several 

 groups and based on one or a few environmental factors is hopeless. 

 Progress in interpreting the distribution of animals can be made only 

 by working out the geographical history of each group separately, and 

 attempting to discover the several factors which have influenced its 

 wanderings. 



References 



Gadow, Hans. The Wanderings of Aninials. 



ScHARFF, R. F. Origin and Distribution of Life in .\merica. 



ScHARFF, R. F. The History of the European Fauna. 



