1914] Grinnell: Mammals and Birds of the Colorado Valley 63 



to be further explained on historical grounds. The two sets of areas 

 thus denned do not by any means correspond. Yet the reviewer 

 cannot fail to note, here and there, places where boundaries coincide, 

 and such coincidences are so frequent as to be suggestive of real con- 

 cordance in some significant manner. Is it not probable that both 

 schools are approximately 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? The opportunity is 

 here taken to attempt to brine' into accord the systems of the two 

 schools. 



The period of field study up to the present time devoted by the 

 writer to the animal life of the climatically diversified state of Cali- 

 fornia has led him to the recognition of three distinct orders of distri- 

 butional behavior as regards terrestrial vertebrates. These are indi- 

 cated in the terms: zonal, faunal. and associational. 



Every animal is believed to be limited in distribution tonally by 

 greater or less degree of temperature, more particularly by that of 

 the reproductive season (see Merriam, 1894). When a number of 

 animals (always in company with many plants similarly restricted) 

 approximately agree in such limitation, they are said to occupy the 

 same life-zone. 



The observation of this category of distributional delimitation 

 is particularly easy in an area of great altitudinal diversity like that 

 comprised in the southwestern United States. The writer is led to 

 wonder if those authors who minimize the importance of temperature 

 have ever been privileged to travel, and carry on field studies, outside 

 of the relatively uniform eastern half of North America ! 



Study of any area which varies widely in altitude and hence 

 provides readily appreciable differences in daily temperature from 

 place to place brings conviction of the very great effectiveness of 

 temperature in delimiting the ranges of nearly all species of animals 

 as well as of plants. Particular attention may be called to the results 

 of a biological survey of Mount Shasta (Merriam, 1899). 



But temperature is not to be considered the only delimiting factor 

 of environment, though its possible overemphasis by the Merriam 

 school seems to have led some other persons to believe that this view 

 is held. In fact, it becomes evident after a consideration of appro- 

 priate data that very many species are kept within geographic bounds 

 in certain directions only by an increasing or decreasing degree of 

 atmospheric humidity (see Grinnell and Swarth, 1913, p. 217). By 



