250 Experimental Zoology 



(3) It seems to me entirely wrong in principle to suppose that 

 any one function of a species can be thought of as separated from 

 all the others and increased or decreased to any extent demanded 

 by the theory of natural selection without altering the other 

 functions of the organism. Even if this could be done to some 

 extent by selection of individual differences, the change would 

 only be temporary and disappear when the selection ceased. 

 No one will suppose, I imagine, that in every generation the 

 length of life of the species in relation to its power of reproduc- 

 tion is being regulated in this way, and, unless selection is in- 

 cessant, little or nothing can be gained, because the function or 

 structure will return to its natural condition if let alone. The 

 length of life of each species is as characteristic of it as any 

 other of its functions, and must have appeared when the species 

 came into existence. 



(4) In regard to Weismann's view as to the origin of death in 

 consequence of the advantage conferred thereby on the species, 

 it is only necessary to point out that if matters of this sort were 

 decided solely on the grounds of the advantage to the species, it 

 might often be of much greater advantage to improve the power 

 to repair the injuries of the adult than to bring in death as a solu- 

 tion of the difficulty. Those animals that have reached maturity 

 after overcoming the dangers of development and growth might 

 be of greater use by living in maintaining the species than by dy- 

 ing, and if death can be determined by selection, so could no 

 doubt the prolongation of the reproductive power (to replace 

 accidental losses in numbers) and the power to repair. 



This whole method of arguing seems to me to be so uncertain 

 that it is unprofitable. What we need is obvious, namely, to 

 study the physiology of the process of senescence in different 

 groups. 



If growth and length of life are simply physiological changes, 

 why cannot the process be maintained artificially at any desired 

 point, or why not reversed and old age grow young again ? We 

 know too little at present regarding the process of growth to 

 make it worth while even to hazard a guess whether the growth 



