AN ESS A Y ON LONGEVITY. 29 



conditions. The close relation of the average lon- 

 gevity to the welfare of the species is seen in cases 

 where man has interfered with this quantity, as in 

 game-preserving. Gamekeepers killed 'vermin,' i.e. 

 hawks, weasels, foxes, &c., which were in the habit of 

 diminishing the average longevity of the grouse, by 

 destroying weakly birds. The vermin being de- 

 stroyed, the average longevity was unduly raised, and 

 as a consequence we had the grouse disease, which 

 threatened the extinction of the species. Other such 

 cases might be actually pointed to, or conjectured.^ 



We have then these three quantities of life — the 

 normal, the absolute, and the average longevity, each 

 one of which, in its unequal distribution, we are en- 

 titled to assume, is fitted to the requirements of the 

 specific organism, either by special design, or by the 

 gradual evolution of relations. By enquiring what 

 the correspondences are, we may endeavour to frame 

 some general propositions as to the causes affecting 

 longevity, and thus be the better able to examine the 

 question of man's longevity. It is seldom, on account 

 of the small knowledge available, that the term 

 ' absolute potential longevity' will have again to 

 be used. In speaking of potential longevity, or 

 longevity, henceforth, unless otherwise said, normal 



' Such, for example, are the diseases of domesticated animals, and of 

 civilized man himself. The incapacity of some plants and animals to 

 become established in a new country may be attributed, in many cases 

 to the absence of some cause — nature's sanatory police — which would 

 check undue average longevity, and thus maintain a healthy stock. 



