1000 



800 - 



600 - 



400 



200 



1000 



_?800 



Q. 



X 



£ 600 



o 

 > 



> 400 



200 - 



1000 



800 - 



600 



400 



200 



-100 -50 50 100 150 200 



PERCENTAGE DEVIATION FROM MEAN DURATION OF LIFE 



Fig. 6.— Types of survival curve (from Pearl, after Allee et al., 1949). 



