190 Shelby D. Gerking 



Senile death in Bidder's opinion should be reserved for those 

 animals which exhibit "negative growth" after full sexual 

 maturity and specific growth have been achieved. In his 

 last paper Bidder took his stand on the basis of correlating 

 ageing with specific growth and ceased to make an issue of 

 reproductive performance. 



The issue of reproduction was soon raised again by Orton 

 (1929) in a somewhat different manner. He asked the question 

 whether or not fishes might die as a direct or indirect result of 

 expending themselves in reproduction. Russell's (1914) data 

 were cited, which suggested that the reproductive organs of 

 large haddock (Melanogr animus aeglefinus) make up a larger 

 proportion of the body weight than they do in smaller speci- 

 mens. Metabolism is concentrated overwhelmingly on repro- 

 duction, and although the expenditure of energy may not kill, 

 the animal may become so unstable that otherwise sublethal 

 factors might be brought into play and cause death. Orton 

 dramatized this effect by terming it "over-reproduction". 



Orton's viewpoint turns up in another connexion. Svardson 

 (1949) considered the effect of natural selection on the egg 

 number of fishes and concluded that there must be opposing 

 selection pressures for decrease as well as an increase in egg 

 number. Clearly, a mutation causing an increase in egg 

 number would have a selective advantage and spread through 

 a population unless there were factors opposing such a change. 

 In his words: "There would' be an anatomical and physio- 

 logical limit for the females' capacity of producing more eggs. 

 When the egg number has been brought up to this limit, only 

 those individuals not exceeding the limit could spawn, while 

 the others died." Svardson later rejects this as a major 

 factor in evolution and concludes that egg number would be 

 limited by the ability of fish to protect the young among 

 those fishes which behave in this way. More importantly, he 

 thinks, egg number is limited by the premium on large eggs 

 which produce large fry. He postulates that the large fry are 

 in a better competitive position in the population. 



