AN UNSOLVED PROBLEM OF BIOLOGY 



wrinkles is therefore simply that, being older, they have 

 frowned and grinned more often. But we must also ask whether 

 the skin of older people more readily takes the impress of 

 creasing and folding. Does Ql first flexure in the skin of an older 

 person leave a bolder trace than a first flexure in the skin of 

 someone younger? We may be certain that it does. But the 

 point is that both an increase in innate susceptibility to wrink- 

 ling and the cumulative effect of recurrent creasing have played 

 a part in the history of wrinkles; and although we can dis- 

 tinguish the two sorts of causes in theory and in experimental 

 practice, they cannot be disentangled merely by contemplating 

 the wrinkle as Sifait accompli. 



Wrinkling is an unimportant example of a kind of disability 

 that affects all animals. Any injury that leaves a physical trace, 

 as all but the most trivial do, increases the vulnerability of 

 older animals, because injuries of one sort or another are 

 recurrent hazards and older animals, having been exposed to 

 them more often, will have built up a bigger actuarial debt. 

 Skin scars may be individually trivial things, but the older 

 animals will have more of them; and apart from that, germs 

 gain easier access to the body during the time taken by a 

 wound to heal. Fractures of bone are slow to reunite and 

 animals make easy prey until they have done so. The height- 

 ened blood-pressure that accompanies the shocks and alarms 

 of natural living predisposes the blood-vessels to degenerative 

 change. Cells may produce faulty copies of themselves in what 

 should be an act of exactly symmetrical division; division is 

 recurrent and faulty copies are perpetuated, so that their ill 

 effects, summed over the cell population of the body, are bound 

 to add up. The efficacy of most of the known cancer-provoking 

 chemical compounds depends upon the repeated exposure of 

 tissues to their action over long periods. Infections are re- 

 current hazards; most infections damage cells, and some do 

 permanent damage of a sort that increases vulnerability in an 



53 



