94 CONTEMPORARY SCIENCE 



the time the egg was laid until the maggot was trans- 

 formed into a chrysalid. The influence was practically 

 identical with that of temperature on the total duration 

 of life. Thus the larval period lasted 5.8 days at 25 C. 

 and 17.8 days at 15 C., a ratio of about 1 : 3. The total 

 duration of life of aseptic flies is 38.5 days at 25 and 

 123.9 days at 15 C., also a ratio of about 1 : 3. We are, 

 therefore, justified in making the statement that the in- 

 fluence of temperature upon the duration of the larval 

 period or the youth of aseptic flies is practically identical 

 with the influence of temperature on the total duration of 

 life. 



Experiments by Uhlenhuth on the influence of tempera- 

 ture on metamorphosis in salamanders have shown that 

 it is similar to that observed in flies. Salamanders kept 

 at 25 metamorphosed when they were n weeks old, 

 while salamanders kept at 15, under otherwise identical 

 conditions, metamorphosed when they were 22 weeks old. 

 All these data suggest the possibility that the duration of 

 life and the duration of the larval period or of youth are 

 in reality times required for the completion of definite 

 chemical reactions. The cessation of respiration leading 

 to the termination of life and the alterations in circula- 

 tion leading to metamorphosis or termination of youth 

 are critical points ; and it seems possible that these points 

 are reached when a certain toxic substance is formed in 

 adequate quantity in the body, or when a necessary sub- 

 stance is destroyed or sufficiently diminished in quantity, 

 or when both conditions are fulfilled. 



We can prolong or shorten the period of youth in 

 amphibians not only by modifying the temperature but 

 by withdrawing or offering the specific substance which 

 causes metamorphosis, namely iodine or thyroid material. 

 There is no end to the substances capable of hastening 

 death. Shall we ever find a substancfe which will pro- 

 long the duration of life? At present we can neither 

 deny nor affirm the possibility. 



