PAPERS ON ZOOLOGY 259 



problems, though perhaps problems of a kind which are 

 common to most animals. 



m. THE EFFECT OF C0:S^DITI0NS OX PHYSIOLOGICAL 

 LIFE-HISTOEIES 



From the standpoint of insect pests such as the cod- 

 lin moth it is important to know how external conditions 

 effect the rate of development, fecundity and length of 

 life of the individual. It has long been known that tem- 

 perature has an important effect. De Candolle, the 

 noted Swiss botanist, stated in 1S30 that the time to ma- 

 turity of wheat, for example, differs with the mean tem- 

 perature above 6'C. So that the "total degree days" or 

 the number of degrees above 6°C multiplied by the num- 

 ber of days has a constant value. When De Candolle 

 stated this he also stated the time temperature curve for 

 the development of a plant is an equilateral hyperbola, 

 the reciprocal of which is a straight line crossing the 

 axis of temperatures at the temperature at which de- 

 velopment does not take place but immediately above 

 which development begins to t^ke place. In the ninety 

 years that have elapsed sinc^ this discovery, this prin- 

 ciple has been discovered repeatedly and announced as 

 new and original. (See Figure 1). It was not until 

 1914 that the Danish physiologist, Krogh. working upon 

 the frog's egg discovered that this law holds good only 

 within a limited range of temperature. He published a 

 list of the various species which had been studied and 

 gave the limits within which the law could be expected 

 to hold. It will be seen from Figure 1, particularly from 

 the reciprocal curve, that development is a little too rapid 

 at the lowest temperatures and considerably too slow 

 at the highest temperatures to conform strictly to the 

 equilateral hyperbola and a fixed total temperature for 

 completion of development. Xevertheless this method 

 constitutes a valuable guide in many kinds of study but it 

 has been found in the course of studies carried out on the 

 chinch bug and codlin moth in the University of Illinois 

 Vivarium under the auspices of the Natural History Sur- 

 vey that many factors other than temperature influence 



