THE INHERITANCE OF DURATION 151 



Others he winds only halfway, and they stop after four 

 days. Again the clock which has been wound up for the 

 full eight days may fall off the shelf and be brought to a 

 stop at the third day. Or someone may throw some sand 

 in the works when the caretaker is off his guard. So, 

 similarly, some men behave as though they had been 

 wound up for a full 90-year run, while others are but 

 partially wound up and stop at 40 or 65, or some other 

 point. Or, again, the man wound up for 80 years may, 

 like the clock, be brought up much short of that by an 

 accidental invasion of microbes, playing the role of the 

 sand in the works of the clock. It is of no avail for 

 either the clock or the man to say that the elements of the 

 mechanism are in whole or in major part capable of fur- 

 ther service. The essential problem is : what determines 

 the goodness of the original winding? And what rela- 

 tive part do external things play in bringing the running 

 to an end before the time w T hich the original winding was 

 good for? It is with this problem of the winding up and 

 running of the human mechanism that the present chap- 

 ter will deal. 



There are two general classes of factors which may 

 be involved here. These are, on the one hand, heredity 

 and, on the other hand, environment, using the latter term 

 in the broadest sense. Inasmuch as we can be reason- 

 ably sure on a priori grounds that longevity, like most 

 other biological phenomena, is influenced by both hered- 

 ity and environment the problem practically reduces itself 

 to the measuring of the relative importance of each of 

 these two factor groups in determining the results we see. 

 But before we start the discussion of exact measurements 

 in this field let us first examine some of the general evi- 



