Conclusion 



which was largely disappointed. If such an adjustment is pos- 

 sible, it is most likely, perhaps, to concern one or more of the 

 anabolism-promoting substances which maintain growth in the 

 young animal. 



To the question 'Can the effective human life-span be pro- 

 longed artificially?' the most probable answer, based on all 

 these possibilities, would appear to be 'Yes'. To the further 

 question 'By what factor?' no meaningful answer can be given 

 until we know more of the nature of the predominant processes 

 which determine human senescence. Supplementary questions 

 dealing with the degree of reversibility in established senile 

 change cannot at present be answered at all, beyond the con- 

 jecture that the morphogenetic programme in man is hardly 

 likely to be simply reversible in any fundamental sense, but that 

 the irreversibility of local changes in ageing is at present prob- 

 ably over- rather than under-estimated. 



The only excuse for such speculation is, in any case, the pos- 

 sibility that it will drive us into the laboratory to ascertain the 

 facts and to answer the questions it raises, thereby removing 

 gerontology from the field of 'entelechies' and 'inherent prin- 

 ciples' into that of intelligible evidence. 



In planning research upon a subject such as senescence, it 

 pays to put to ourselves the questions which most urgently 

 require to be answered, and then to select from the list in the 

 likely order of practicability. From the defects which exist in 

 the evidence, the essential preliminary questions concerning 

 senescence which we must ask appear to be these. 



(1) Does senescence occur in all vertebrates? 



(2) In what instances, if any, among vertebrates does sen- 

 escence coexist with continuing somatic growth, or non- 

 senescence with fixed size? 



(3) To what extent is arrest of developmental processes com- 

 patible, at different stages of vertebrate ontogeny between 

 conception and senility, with activity and normal func- 

 tion? Is the retardation of further development, towards 

 senility, realizable after puberty in mammals? 



(4) To what extent is the artificial induction of somatic 

 growth possible in the mature vertebrate, and what is its 

 effect on the specific age? 



197 



