AN UNSOLVED PROBLEM OF BIOLOGY 



there be about the members of what we are now obhged to call 

 the under-privileged classes? There is still, it appears, no more 

 to be said about senescence in fish than was said by my pre- 

 decessor Sir Edwin Ray Lankester some eighty years ago: 

 •"Fish are not known to get feeble as they grow old, and many 

 are known not to get feebler/ My professional colleagues will 

 know that Dr G. P. Bidder held some fascinating and far from 

 implausible views on the origin of senescence which turn on the 

 belief that fish do not deteriorate with ageing. These I cannot 

 delay with. But is it not a most revealing fact that there should 

 be any doubt about the matter at all? Fish ma7/ be potentially 

 immortal. in the sense that they do not undergo an innate 

 deterioration with ageing, and yet the naturalists who ought 

 to know about it simply can''t be sure! As you will see, this 

 uncertainty is the most tell-tale evidence in favour of my later 

 argument. Whether animals can, or cannot, reveal an innate 

 deterioration with age is almost literally a domestic problem; 

 the fact is that under the exactions of natural life they do not 

 do so. They simply do not live that long. 



VI 



I have deliberately spent more than half my time in discus- 

 sing the measurement and definition of senescence, and I now 

 want to discuss the factors that may have played their part in 

 its origin and evolution. As a text I shall use a quotation from 

 the works of August Weismann. 



Death takes place because a worn-out tissue cannot for ever 

 renew itself. Worn-out individuals are not only valueless to the 

 species, but they are even harmful, for they take the place of those 

 which are sound ... by the operation of natural selection, the life 

 of a theoretically immortal individual would be shortened by the 

 amount which was useless to the species. 



Weismann''s propositions have the great merit of suggesting, 



57 



