DISHARMONIES AND OTHER SHADOWS 593 



some organisms in which rejuvenescence keeps pace with 

 senescence, and natural death is evaded. At the top of the 

 scale there is the senility of many men and of some domestic 

 animals, like horses and dogs. It is certain that senility is 

 not within the scheme of Animate Nature apart from Man. 

 For many wild animals there is normal senescence, for many 

 there is not even that. There is a slight lowering of vitality 

 and a slight environmental buffet sends them off the stage. 

 But why is it that the fish Aphia pellucida lives only for a 

 year, dying off like an annual plant, while others live for 

 many years? The probability is that the duration of life 

 is limited to some extent by the constitution of the creature, 

 but that within these limits it has been regulated in adap- 

 tation to the conditions of life, that it has been punctuated 

 in reference to large issues, namely the welfare of the species. 

 Not that there is any purposive adjustment, but simply that 

 for each set of given conditions there is an effective age 

 which becomes the age of the surviving types. It is not 

 difficult to understand that a variation in the direction of 

 longevity might be very unprofitable and would be certainly 

 eliminated by the gradual disappearance, paradoxical as it 

 may seem, of the long-lived type. For the longevity might 

 mean that the organism continued multiplying when it was 

 past its best and thus impaired the vigour of the stock. The 

 longevity might mean that the organism continued multi- 

 plying after it had suffered so many dints from the years 

 that it could no longer give the offspring a successful send- 

 off in life. Such variations condemn themselves literally, 

 and the length of life is by selection adaptively punctuated 

 towards the welfare of the race. In some of the higher or- 

 ganisms prolonged multiplication is constitutionally pre- 

 vented on the female side after a certain age is reached, and 



