



ILLUSTRATIONS 



1 Annual rate of mortality per 1,000 by sex: United States, 

 1939-41 15 



2 Number of survivors out of 100,000 born alive, for each 

 race by sex: United States, 1939-41 15 



3 Number of Survivors out of 100,000 male live births, from 

 recent life- tables for selected countries 16 



4 Frequency distribution of ages at death in a cohort starting 

 with 100,000 live births, based on the mortality of white 

 males: United States, 1939-41 16 



5 (a) Survival curve at a constant rate of mortality 19 

 (b) Survival curve of a population which exhibits sene- 

 scence 19 



6 Types of survival curve 20 



7 Survival curves of a German population 22 



8 Survival curves for cafeteria tumblers 23 



9 African Ibis (Threskiornis aethiopicus) . Survival of 21 indi- 

 viduals 27 

 Night Heron (Nycticorax nycticorax). Survival of 17 indi- 

 viduals 27 



10 Orkney Vole (Microtus orcadensis) . Survival of 24 individuals 27 



11 Patagonian Cavy (Dolichotis patagona). Survival of 55 

 individuals 28 



12 Mouflon Sheep (Ovis musimon). Survival of 77 individuals 28 



13 Irish Wolfhounds. Survival of 67 individuals from 12 

 months of age — sexes combined 29 



14 The decline in egg production in successive years of laying 



— domestic fowls 63 



15 (a) Growth in length of male fish of the genera Xiphophorus, 

 Lebistes and Heterandria during the first year of life 66 

 (b) Growth in length of female fish of the genera Xipho- 

 phorus, Lebistes and Heterandria during the first year of life 66 



16 Growth of trout in Windermere and the small tarns 71 



xi 



