LAW OF FUTURE SPECIFIC AND SOCIAL EFFICIENCY 257 



processes take place. Animals characterized by active and restless 

 mobility are said to possess a short life, while inertness, such as that of 

 the Amphibia, is accompanied by relatively great length of life. These 

 statements are, however, invalidated by the fact that birds as a general 

 rule possess great mobility and at the same time great length of life. 



The true answer to the problem seems to lie in the fact that 

 duration of life is really dependent upon adaptation to external con- 

 ditions, that its length whether longer or shorter, is governed by the 

 needs of the species, and that it is determined by precisely the same 

 mechanical process of regulation as that by which the structure and 

 function of an organism are adapted to its environment. Weismann 

 writes, "Assuming for the moment that these conclusions are valid, 

 let us ask how the duration of life of any given species can have 

 been determined by their means. In the first place, in regulating 

 duration of life, the advantage to the species, and not to the indi- 

 vidual, is alone of any importance. This must be obvious to any one 

 who has once thoroughly thought out the process of natural selec- 

 tion. It is of no importance to the species whether the individual 

 lives longer or shorter, but it is of importance that the individual 

 should be enabled to do its work towards the maintenance of the 

 species. This work is reproduction, or the formation of a sufficient 

 number of new individuals to compensate the species for those which 

 die. As soon as the individual has performed its share in this work 

 of compensation, it ceases to be of any value to the species, it has 

 fulfilled its duty and may die. But the individual may be of ad- 

 vantage to the species for a longer period if it not only produces 

 offspring, but tends them for a longer or shorter time, either by pro- 

 tecting, feeding or instructing them. This last duty is not only 

 undertaken by man, but also by animals, although to a smaller 

 extent; for instance, birds teach their young to fly, and so on. 



" We should therefore expect to find that, as a rule, life does 

 not greatly outlast the period of reproduction except in those species 

 which tend their young; and as a matter of fact we find that this is 

 the case."(') 



(1) Weismann, Essays on Heredity, Vol. I., p. 10. 



