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 
