The Biology of Senescence 



age, beyond the inevitable increment from accumulation of 

 evident injuries. He suggested that vertebrate senescence is a cor- 

 relate of the evolution of determinate growth and of a final absolute size. 

 Bidder regarded determinate size as a property which had 

 evolved as a result of the migration of vertebrates to dry land. 

 He pointed to a number of instances in fish where constant 

 expectation of life, capacity for growth, and general vigour 

 appeared to persist indefinitely (Bidder 1925). Bidder's argu- 

 ment is of importance, and is worth quoting in full. 



'Giant trees, cultures of chick cells and of Paramecium, 

 measurements of plaice and of sponges, all indicate that indefi- 

 nite grow this natural. Galileo proved it fatal to swiftly moving 

 land animals, therefore swiftly moving mammals and birds 

 were impossible until their ancestors had evolved a mechanism 

 for maintaining specific size within an error not impairing 

 adequate efficiency. Even without evidence of evergrowing 

 organisms, we could not suppose that the close correspondence 

 to specific size, which we see in all swiftly moving creatures of 

 earth or air, results from mere "senescent" fading-out of the 

 zygotic impulse to cell division and cell increase. Specific size 

 is probably most important to birds, with their aeroplane 

 mechanics stricdy enjoining conformity of scale to plan; but to 

 men it is most noticeable in man. Only familiarity prevents 

 marvel at the rarity of meeting a man more than 20 per cent 

 taller or shorter than 5 \ ft., or of discovering his remains in any 

 place, or any race, or any epoch. Probably our erect posture 

 enforces accurate proportions of length to weight, for running. 



'Adequate efficiency could only be obtained by the evolution 

 of some mechanism to stop natural growth so soon as specific 

 size is reached. This mechanism may be called the regulator, 

 avoiding the word "inhibitor" so as not to connote a physio- 

 logical assumption. However ignorant we are of its nature, its 

 action is traced in anthropometric statistics; a steady diminu- 

 tion in growth rate from a maximum at puberty to a vanishing- 

 point in the twenties. That the regulator works through change 

 in the constitution of the blood is shown by the perpetual divi- 

 sion of Garrell's chick cells in embryonic plasma, whereas cell 

 division is ended in the heart of a hen. 



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