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THE AMERICAN NATURALIST [Vol. LI 



American continent, one single factor does happen to loom up 

 as being the most frequent delimiter of distribution, or even the 

 ultimately effective one, in greater or less degree, even though 

 other factors be effective also. This factor is temperature. 

 The cases cited illustrate the tenet that in some direction or an- 

 other, temperature beyond certain limits, up or down, cuts off 

 further dissemination. This is part of the basis of the life-zone 

 idea. But, as I have tried to bring out above, this fact is in no 

 way antagonistic to the claim that other factors, as of humidity, 

 food supply, and shelter, also figure critically, giving a basis 

 for recognizing faunal areas and associations. Finally, if our 

 discussion of the subject has been sound, it is evident that data 

 secured through field observation can be so employed as to bring 

 results essentially similar to, and as conclusive as, those secured 

 through laboratory experimentation. 



Joseph Grixnell 



Museum of Vertebrate Zoology, 

 November 1, 1916 



