CHAPTER XXVI 



ANIMAL DISTRIBUTION 



(By Willis Hewatt, Texas Christian University) 



Life Regions and Zones of the Earth 



Every area of the earth has its animal and plant life. The ap- 

 parently barren sun-baked desert, the ice-capped polar regions, the 

 highest mountain tops, and the tropical rain forests are all inhabited 

 by their faunas and floras. Life in the seas extends from the shore 

 line to the greatest depths. After extensive taxonomic surveys have 

 been made in practically all areas of the earth and these data are 

 studied, it is found that the earth can be divided into fairly definite 

 horizontal regions and vertical zones. These two phases of the dis- 

 tribution of organisms on the earth are very closely related to each 

 other but are usually studied separately as geographic or horizontal 

 distribution and hathymetric or vertical distribution, respectively. 



Geographic Distribution. — Many attempts have been made to 

 divide the earth into horizontal life regions based upon the distri- 

 bution of various groups of animals and plants. The exact bound- 

 aries of the regions are not fully agreed upon since several groups 

 of animals have been used as criteria for the division into life re- 

 gions. P. L. Sclater (1829-1917), the earliest zoogeographer, worked 

 with the perching birds. These forms were well adapted to such a 

 study since they have little power of flight, they are widely dis- 

 tributed, and they had been very closely studied. The early (1876) 

 classic work of the Englishman, Alfred Russell Wallace, in which 

 the earth was divided into six primary regions, is the one most com- 

 monly followed by modern biologists. Wallace based his divisions 

 upon the distribution of the mammals. Other significant studies in 

 geographic distribution have been based upon the geographic ranges 

 of mollusks, earthworms, moths, butterflies, spiders, fresh-water fishes, 

 reptiles, and many others. From works dealing with such varied 

 groups of animals it is readily understood that any classification of 

 the regions of the earth will depend greatly upon the group of ani- 

 mals used as a criterion. On the other hand, there are many corre- 

 lations found in the studies of the geographical ranges of all of these 

 groups of animals. The zoogeographical regions of Wallace and the 



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