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THE AMERICAN NATURALIST 



[Vol. LII 



that the completed sexually mature individual can not 

 be produced oftener than usually obtains. There is, to 

 be sure, much evidence that the tendency to hibernate 

 is not very firmly established in some species and that 

 under stimulation animals may be induced to repro- 

 duce nearly continuously, at least for a number of gen- 

 erations. Cessation of development in any given case is 

 as much attributable to some factor falling below the 

 threshold of development as to heredity. The environ- 

 ment is extremely complex, and the number of factors 

 which may cause cessation of development and which have 

 been already established, are so numerous as to indicate 

 that the number is very much greater than is commonly 

 supposed, including temperature, moisture, light, oxygen, 

 evaporation, quantity of food, or absence of any one of 

 many necessary food constituents. These appear to 

 operate in accordance with the law of toleration (Shel- 

 ford, '13) and, with respect to food, in accord with Leibig's 

 law of minimum. Where dormant periods are well estab- 

 lished, their occurrence with reference to the usual sea- 

 sonal rhythm makes any modification of the usual life his- 

 tory difficult or impossible. 



Variations from the "normal" seasonal weather, and 

 weather changes are of especial interest as modifying the 

 usual seasonal succession of adult animals or any area. 

 In springs with unusually prolonged cool weather, the 

 various pond species, such, for example, as those noted 

 on page 146, are crowded together, and reach maturity 

 much more nearly at the same time than in normal sea- 

 sons. The same phenomenon has been observed by the 

 writer in the case of the flowering of early spring plants 

 of an area near Chicago. The differences in the 

 of different species to the same conditions show their dif- 

 ferent physiological constitutions. This type of varia- 

 tion indicates that such maladjustments as resulted in the 

 depletion of the grain crop by the grain aphis in the 

 southern part of the wheat belt, because the weather fa- 

 vored them, may occur in undisturbed localities, though 



