The Distribution of Senescence 



neatness with which its growth can be controlled by varying 

 the food intake and living space. 



A large population of Lebistes has been kept at University 

 College under conditions in which each individual was isolated 

 throughout life, and the dates of birth and death were accur- 

 ately known. Dr. H. Spurway has kindly allowed me to use her 

 data on this population; from these, and from fish which I have 

 kept myself, it has been possible to construct a number of life- 

 tables for Lebistes under different conditions. These are con- 

 sistent, and all show a progressive increase in mortality with 



100 



DAYS 



100 



500 



1000 



Fig. 18a. — Survival of 12 male and 40 female guppies (Lebistes 

 reticulatus) kept individually at 25° C: sexes combined. (From 

 data provided by Dr. H. Spurway.) 



advancing age very similar to that found in mammalian life- 

 tables (Fig. 18fl). The fish in this sample were not measured 

 during life, but had certainly reached or exceeded the usual 

 'specific size'. The growth rate of female Lebistes and the size 

 at which the growth curve reaches a plateau can very easily be 

 altered. Spurway has found that these fish could be kept in 

 individual half-pint milk bottles — by combining restricted space 

 with restricted diet, I have kept female Lebistes at a length of 

 about 2 cm. for as long as 600 days. In this state they are 



75 



