The Biology of Senescence 



reproductively mature (unlike the rats subjected to retardation 

 by McCay — p. 149) and capable of resuming growth. There are 

 'specific sizes' characteristic of each size of container and each 

 level of nutrition — or, alternatively, of each population-density 

 in a tank, when a fish is promoted from one such container to 

 a larger, or when fish are removed from a tank population, a 

 new plateau is rapidly reached. The curve given by Wellensieck 

 represents only one such equilibrium, The growth capacity also 

 appears to decline somewhat throughout life, and there is a 



A = half pint milk bottle, restricted food 



D = 21b jar, restricted food 



O = 7 lb jar, plentiful food 



• = large aquarium tank, plentiful food 



AGE (days) 



100 



200 



300 



400 



500 



600 



Fig. 18£. — Growth of female guppies (Lebistes 

 reticulatus) in different conditions of feeding and 

 living-space. The symbols indicate the time of 

 transference to new conditions. All the fish 

 were alive at the time of drawing. The growth 

 pattern of the fish whose life-table is given in 

 Fig. 1 8a approximated to that of the fastest 

 group shown in this figure. 



practical limit, as might be expected, to the size of guppy which 

 can be produced at maximum food intake and maximum living 

 space. The combination of variables in Lebistes, and the fact 

 that the life-span of the non-growing males is not, upon present 

 data, grossly different from that of the growing females, suggest 

 that a great deal about growth and senescence in fish can be 



76 



