130 THE AMERICAN NATURALIST [Vol. LII 



undertake to discuss them without apology. The prac- 

 tical significance of variations to agriculture is shown by 

 the destruction of the wheat crop in the Southwest by the 

 wheat aphis. This was due to differences in response to 

 weather on the part of the pest as compared with its ene- 

 mies. The cause of seasonal appearance, or more espe- 

 cially of variations of seasonal appearance, is to be 

 found in the influence of external factors on the initiation 

 and velocity of growth and on fecundity and length of 

 life, in dormancy in various stages in the life histories, 

 and in the adjustment of the innate rhythm to the annual 

 climatic cycle. 



II. The Influence or External Conditions on Time or 

 Appearance and Number of Individuals 



1. Differences in Initiation and Velocity of Develop- 

 ment,— The problem of initiation of development is one 

 that has attracted much attention of late on account of the 

 importance of an ability to predict the time when various 

 insect pests will emerge from hibernation or will reach a 

 stage of development at which it is necessary to spray, 

 if such treatment is to prove satisfactory. 



In this connection attention has been directed to the 

 conditions, particularly of temperature, under which 

 there is no development during periods lying within the 

 bounds of the ordinary life history of the animal in ques- 

 tion (Sanderson, '10, Headlee, and Peairs). The limit at 

 which development does not take place, usually called 

 physiological zero or zero of development, is better termed 

 tlm shnld of development. Sandersonhas discussed vari- 

 ous data and theories relative to the effect of temperature 

 on development. 



The attention of physiologists has been directed toward 

 the study of the effects of temperature on the rate of 

 metabolism and development. In general the results 

 of such study have been interpreted with reference to 

 Van't Hoff's law relative to the increase of reaction 

 with a rise of temperature of 10 degrees, usually des- 



