8o2 LIFE : OUTLINES OF GENERAL BIOLOGY 



therefore the fox and the hare do not require to Hve so long as 

 the eagle. 



It is not for the welfare of the species that an animal should 

 continue to multiply when it is past its best — that is the one limit ; 

 but it is not for the welfare of the species that the birth-rate should 

 fall below the death-rate when food is abundant and the area not 

 overcrowded — that is the other limit. In reference to these issues 

 the length of life has been regulated by selection. 



The illustration just given indicates the shrewdness of Weismann's 

 theory that the duration of life has been determined by Natural 

 Selection; yet we return to our criticisms, (i) that the same result 

 might be reached if the Golden Eagle increased the number of eggs 

 in its clutch, or had two families in the summer — both frequently 

 variable characters among birds; and (2) that the process of selec- 

 tion indicated by Weismann would not operate in regard to the ten 

 years of the Golden Eagle's pre-re productive life. But our radical 

 objection to the theory as it stands, still is that it takes too little 

 account of the constitution of the organism. It is eas}^ to say that the 

 organism must live longer if it is to meet by its reproductivity the 

 chances of death; but all things are not possible to all sorts of 

 organisms. Thus, without rejecting the view that the duration of 

 life is punctuated by natural selection in relation to the needs of the 

 species, we plead for a recognition of the existence of long-lived 

 and short-lived physiological constitutions. Selection can only 

 operate within limits — limits of constitution, an established meta- 

 bolic ratio. The long-lived types stave off the ageing or senescence to 

 which all complex animals are liable; but for the short-lived types 

 this is not possible. But how can an organism stave off natural 

 senescence? They may have a highly-efficient regulatory system 

 (hormonic in particular), which harmonises or orchestrates the 

 bodily activities; they may have great capacities for recuperation, 

 by means of long rests, perfect sleep, and frequent changes of food 

 and environment ; they may have evolved a power of resistance to 

 frequently recurrent vicissitudes; and they may have a mode of 

 reproduction which is not so physiologically costly as that of lam- 

 preys, or butterflies, or Century Plants. These qualities are no doubt 

 themselves the outcome of the selection of variations, but they 

 afford material on which the subtler punctuation of life may operate. 

 In the case of man we must clearly distinguish between the 

 average species-longevity, still only about forty years in Europe 

 generallv, but raised to nearly fifty in Britain during the past 

 century or less — but happily raisable substantially further with 

 decreasing infantile mortality, improved sanitation, decreasing 

 warfare, increasing temperance and carefulness, — and the potential 

 species-longevity, which for our present race seems normally between 

 seventy and one hundred years. There is no warrant for fixing any 



