546 DR. r. CHALMERS MITCHELL ON LONGEVITY AND 



addition to knowledge. Having distinguished between potential 

 and average specific longevities, and shown tha,t the latter were 

 determined to a large extent by accidents outside the constitution 

 of the species (destruction by enemies, diseases and so forth), he 

 attributed the former to constitutional causes of which the most 

 important were the degi-ees of evolution or individuation and the 

 amounts of personal and generative expenditure. High individua- 

 tion, and low expenditure were to be associated with great potential 

 longevity. 



A. Weismann, in his famous essay lleher das Dcmer des Lehens 

 (translated in Essays upon Heredity, Oxford 1889). examined the 

 various constitutional explanations of the duration of life. He 

 admitted that small animals might be expected to run through 

 the cycle of life more rapidly than large animals ; that if the 

 period of growth were long it might expand the total duration 

 of life ; that creatures in which the metabolic processes were 

 extremely active, might finish their career more quickly (as Lotze 

 suggested in his Microcosmus) than slow-living creatures ; that 

 the rates of personal and of reproductive expenditure had some 

 influence. But he urged that the application of these various 

 principles was only partial and led to so many inconsistencies 

 that no constant correlation could be established. He came to 

 the conclusion that duration of life was really dependent upon 

 adaptation to external conditions, that its length was governed 

 by the needs of the species and was regulated by the same process 

 as that by which the structure and the other functions of an 

 organism were adapted to the environment. 



In adaptation to the environment, Weismann urged, it is the 

 prosperity of the species and not that of the individual that is 

 concerned. That species is most successful which contains at any 

 time the largest number of vigorous adults, and as every organism 

 in the vicissitudes of life becomes to a cei-tain extent worn and 

 dilapidated, it is not to the advantage of the species that indi- 

 viduals should live too loug. As soon as the business of repro- 

 duction has been successfully accomplished, the advantage of an 

 individual to the species is gone, and the sooner it disappears the 

 better. Animals are in fact wound up to go for the requisite 

 time, and no longer. The mechanism Weismann suggested was a 

 limitation of the possible number of cell generations, admitting 

 that this was hypothetical. The requisite length of time was 

 determined by the reproductive habits of the animals. Slow 

 breeding, for instance the production of a small number at a 

 birth or in a season, long brood-care either embrj^onic or post- 

 embryonic, and all the various circumstances by which an organism 

 or pair of organisms require longer time to replace themselves 

 by their younger and fi'esher children, stretched out the dura- 

 tion of life ; whilst rapid reproduction, absence of brood-care and 

 so forth, contiacted it. By the process of natural selection the 

 incidence of death was adapted to the needs of the species. 



