426 DR. p. CHALMERS MITCHELL OX LONGEVITY AND 



animals, or in animals in captivity, is therefore of scientific and 

 practical intereist. From the scientific point of view, it is the 

 only mode of getting any kind of information as to constitutional 

 differences in longevity and viability. From the practical point 

 of view, it is the only way of judging of the efiect on duration of 

 life of the difierent kinds of environment to which animals may be 

 subjected. There exists as yet very little recorded information, 

 and I hope that this memoir may at least stimulate enquiry and 

 make possible the collection and comparison of data. 



The normal annual death-rate of the human inhabitants of 

 London is about 14 or 15 per thousand, or 1"5 per cent, per 

 annum, that is to say for every two hundred inhabitants there are 

 about three deaths in the year. There is no census of the sparrow 

 ■population of London, but the nvimbers of these hardy birds do 

 not appear to increase or decrease in any marked way. Now 

 a pair of sparrows may rear two or three clutches in the course of 

 the year, each clutch having five or six eggs. If every pair of 

 sparrows alive in London at the beginning of the breeding-season 

 displayed maximum fertility, a death-rate of about 90 per cent, 

 per annum would be necessary to keep the sparrow population 

 stationary. No doubt many sparrows fail to mate successfully, but, 

 judging from the numbers of young birds, it seems highly pro- 

 bable that the annual produce is a least a hundred per cent., which 

 implies a normal death-rate, in a stationar}- population, of at least 

 50 per cent. On January 1, 1910, the number of vertebrate 

 animals in our Gardens was 3186, and during the year, 2354 were 

 added to the collection, making a total population of 5540. In the 

 same period there were 1554 deaths, that is to say, the death-rate 

 was about 28 per cent. Here, then, are three widely diflferent 

 rates, an actual percentage of 1*5 in the case of human beings, an 

 estimated percentage of 50 in sparrows, and an observed percent- 

 age of 28 in the assemblage of vertebrates in the London 

 Zoological Gardens, It is plain that the rates are not comparable ; 

 two of them refer to particular species, the third is a composite 

 made up from many diflferent species of mammals, birds, reptiles, 

 batrachians, and fishes, each with their own difierent rates. The 

 composite percentage is an arithmetical figure as meaningless as 

 would be the sum of so many cows, coins and cabbages. Even in 

 the case of a single species, however, a generalized death-rate is 

 complex and misleading. The human death-rate is nearly 20 per 

 cent, in the first year of life, falls rapidly for the first two or three 

 years, and then more slowly till it reaches a minimum, and then 

 gradually rises as advanced age is reached. The tables of 

 " expectation of life," calculated for the Life Assurance Companies, 

 show that the male infant under a year old has an expectation of 

 41 years, that this expectation I'ises to a maximum of 51 in the 

 fourth year, and then gradually and almost without ii-regularity 

 decreases until the age of one hundred years has been attained. 

 It is clear that in the case of the vast majority of animals, and 



