Nos. 622-623] MIGRATION A FACTOR IN EVOLUTION 479 



with the suggestions of Blackman and Smith ('11) who 

 show that certain physiological processes are better ex- 

 plained as the " result of interacting limiting factors than 

 by the conception of the optima." This principle an 

 extension of the law of the minimum and is formulated 

 by them as follows (p. 411) : 



The identification of the particular limiting IWior in ;iny .iHh.ii.j 



principles. When the magnitude of a function is limited by one of a 

 set of possible factors, increase of that factor, <xnd of that one alone, 



tion ... (p. 397). When several factors are possibly controlling a 

 function, a small increase or decrease of the factor that is limiting, and 

 of that eactor only, will bring about an alternation of the magnitude of 

 the functional activity. 



Probably this formulation should be broader, and be 

 made to include not only a single factor, but all unfavor- 

 able or limiting factors, as I have indicated above, and as 

 both Livingston ('17, p. 8) and as Hooker ('17, p. 201) 

 suggest. 



Kecent additional physiological evidence of the con- 

 centric zonation (gradation) of the limiting factors of 

 temperature and humidity have been made by Pierce 

 ( '16) . He accepts the older idea of the optimum and thus 

 certain of his results on zonation harmonize with my 

 statement of 1904. He shows that for the cotton boll 

 weevil there is a vertical temperature gradient which in- 

 fluences the metabolism, growth and other activities, and 

 that for a given temperature there is a corresponding 

 horizontal humidity gradient which forms concentric 

 zones of less favorable conditions. These extend from 

 the optimum, through dormancy, on to death. It seems 

 likely, however, that the idea of ''interacting limiting 

 factors" explains his facts better than that of the 

 optimum. 



The idea of limiting factors in experimental work is 

 now building up a laboratory idea of environmental com- 

 plexity, even under controlled conditions, which corre- 

 sponds closely with what the field ecologists have called 



