RELATIVE VIABILITY IN MAMMALS AND BIUDS. 427 



notably in the case of wild animals that have been captui'ed and 

 brought into captivity, we have but vague materials for the con- 

 struction of tables of expectation of life or for attaching any value 

 to death-i'ates. We know roughly that theie is an infantile 

 period of low viability from the close of which through the periods 

 of youth, maturity and senescence, there is a gradually decreasing 

 expectation of life. But there are very few cases in which the 

 durations of these periods or the potential duration of the whole 

 life are known accurately, and still fewer cases in which the 

 position of any single individual on its cycle of life can be deter- 

 mined accurately by physical signs, at least in the living ammal. At 

 present there is not enough knowledge on these matters to i-ender 

 possible the construction of any standards against which particular 

 cases could be measured. 



Fortunately there remains a mode of estimating the durations 

 of life in a collection of animals which affords some indications of 

 the total effect of the environment on the duration of life. In 

 1870 Sir Ray Lankester * (whose intellectual acumen has un- 

 ravelled so many tangles in biology) drew some important 

 distinctions in the significance of the word longevity. Strictly 

 speaking, longevity denotes the duration of life of an individual ; 

 in practice, the word connotes some idea of relatively long diu^ation 

 and is used to indicate the duration of a life that has extended 

 to its natural limit. A further distinction is necessaiy. ' The 

 natural limit to the longevity of the individuals of a species may 

 be taken to mean the average age attained by the normal members 

 of a species living under the conditions to which they have 

 become adapted by nature. The specific longevity would be the 

 expectation of life at birth of a normal individual of the species. 

 It is determined partly by the constitution, but still more by the 

 accidents, enemies, diseases, and other external conditions to which 

 the members of the species are naturally subject in every stage 

 of their existence, and it is modified by the powers of evasion, 

 protection, and resistance which they have acquired. On the 

 other hand, it is possible to suppose that a member of a species, by 

 good fortune or by artificial interference, has been removed from 

 the hardships natural to its lot, and placed in an environment 

 relatively ideal ; under such conditions it would survive the 

 specific longevity and attain an age which Lankester called the 

 potential longevity. If we regard the population of Great Britain 

 as living in an environment to which it has been natvirally 

 adapted, and the inhabitants of Great Britain as representing 

 a species, then their specific longevity, the expectation of life at 

 birth, is somewhere about 50 years. What the potential longevity 

 is we do not know, but it is certainly moi'e than a hundred years. 

 The disparity is probably much greater in other cases. In the 

 case of the sparrow the specific longevity, the expectation of life 



* On Comparative Longevitj' in Man and the Lower Animals : London, 1870. 



