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THE AMERICAN NATURALIST [Vol. XLVIII 



known to be subject to sporadic wanderings on the part of indi- 

 viduals, but the species is kept in set bounds by some potent but 

 invisible set of factors. The very fact that individuals are quite 

 capable of temporarily transgressing these bounds and yet do 

 not overstep them en masse emphasizes all the more the remark- 

 able potency of this category of barriers as regards species and 

 higher groups. 



Our geographic studies lead us to designate among these rela- 

 tively intangible barriers: (1) increase or decrease in prevailing 

 temperature beyond certain critical limits, according to the species 

 concerned; (2) increase or decrease in prevailing atmospheric 

 humidity beyond certain limits; (3) modification in food-supply 

 and appropriate breeding and foraging ground. The limits set 

 by each of these factors will vary with the physiological pecul- 

 iarities of the organism considered; in other words the inherent 

 structural equipment of each animal figures importantly. In 

 these three sorts of barriers will be recognized what have been 

 called "zonal.'' " faunal" and " assoeiational" delimitation, each 

 of which I will now try to define. 



Two schools of faunistie students are represented among Amer- 

 ican zoo-geographic writers of the present day. One, of which 

 C. H. Merriam is the most prominent exponent, sees in tempera- 

 ture the chief cause controlling distribution, and deals with the 

 ranges of species in terms of "life /.ones." The other school, of 

 which C. C. Adams, A. G. Ruthven and Spencer Trotter are 

 active advocates, assigns to temperature but a minor role, look- 

 ing rather to a composite control, of many factors, resulting in 

 ecologic "associations," of which plants are essential elements, 

 and which are to be further explained on historical grounds. 

 The two sets of areas thus defined do not by any means corre- 

 spond. Yet the reviewer can not fail to note, here and there, 

 places where boundaries coincide, and such coincidences are so 

 frequent as to be suggestive of real concordance in some signifi- 

 cant manner. Is it not probable that both schools are approxi- 

 mately correct, the difference in mode of treatment being due to 

 different weights given the different kinds of evidence, or, in 

 other words, to difference in perspective? 



Every animal is believed to be limited in distribution zonally 

 by greater or less degree of temperature, more particularly by 

 that of the reproductive season. When a number of animals 

 (always in company with many plants similarly restricted) 



