8oo LIFE : OUTLINES OF GENERAL BIOLOGY 



in his essay On the Duration of Life (1881), to the conclusion that this, 

 like size, is an adaptive character, gradually defined by selection in 

 relation to the external conditions of life and the needs of the 

 species. Given a certain rate of reproduction and a certain average 

 mortality, the duration of life that becomes established will be that 

 which is fittest to secure the survival of the species in the existing 

 conditions. In other words, those species or varieties survive which 

 have attained to a viable correlation between the length of their 

 effective reproductive life and the average mortality. It would 

 handicap a species or variety if its members produced numerous 

 offspring after they had ceased to be effective parents, and if the 

 number of these weaklings and defectives went far in excess of the 

 chances of death. 



There are three considerations that seem to us to show that 

 Weismann's theory requires to be supplemented. The first is that he 

 does not allow enough for the constitution of the organism. It is not 

 every kind of organism that could lengthen out its life-tether to 

 meet the demand for a sufficient number of offspring to balance 

 the chances of death. Secondly, the situation might be saved 

 if the organism increased its fertility without lengthening its 

 life. Thirdly, the process of selection, which Weismann postulated, 

 for the adjustment of the length of life to the needs of the species, 

 would operate through the elimination of variants which con- 

 tinued their reproductive period beyond the time of effective 

 parentage, or else would shorten their reproductive period so 

 much that the birth-rate would cease to balance the death-rate: 

 so in either case there would tend to be automatic elimination. 

 But in many organisms the total duration of life is long because 

 of the extension of pre-reproductive periods — an extension which 

 would not be affected by the selection process on which Weismann 

 relied. 



Again, an animal after the reproductive age of vigour may 

 continue its life by a change of habits, as of feeding, etc. This may 

 be the case with "man-eaters" in India, which are known to be often 

 old and mangy. 



Diversity of Age among Animals. — Let us consider, then, 

 whether any constitutional reasons can be found for the great 

 diversity in the ages that different kinds of animals may attain. 

 The difficulty in finding an answer is largely due to the paucity of 

 secure data. From records of captive animals we know that an 

 elephant may live two hundred years, a pan*ot eighty, a sea- 

 anemone sixty-six, a golden eagle fifty, a toad forty, a crayfish 

 twenty, a blackbird eighteen; but we cannot assume that these 

 figures hold for the same animals living in natural conditions. In 

 some cases the captive animals may live longer, in other cases 

 shorter, than their relatives in freedom. The data are very uncertain. 



