HUMAN LONGEVITY 63 



classes of human beings diflPerentiated from each other in their 

 innate endowments in respect of inherent viability, one class 

 being short and the other being long of this important quality. 

 The second suggested conclusion is that improving the environ- 

 mental circumstances of living can do, and has done, a great deal 

 more for the first class than for the second in the way of in- 

 creased longevity. It appears probable that there is now, and 

 always has been in past ages, a class of human beings by nature 

 so abundantly endowed in the matter of viability that they 

 have always, as a statistical group, so nearly realized their 

 innate potential viability regardless of environmental circum- 

 stances as to be not significantly affected in average duration of 

 life by any general improvement of those circumstances. 



Detailed study of the life histories of extremely longevous 

 persons, such as has been possible with our collection of such 

 records, strongly suggests that nonagenarians and centenarians 

 are biologically differentiated from the general run of mankind 

 in just the manner postulated for the second biological class 

 just described. As a group nonagenaria'ns and centenarians have 

 definitely not led protected lives in specially favorable environ- 

 mental circumstances; nor have they had better medical advice 

 or care than the generality of menj nor, finally, have they con- 

 ducted their lives more hygienically than others, according to 

 the rules and precepts generally regarded as conducive to long 

 life. On the contrary they have just lived, but lived a much 

 longer time than most. 



V 



So far the discussion has been almost entirely of the innate, 

 constitutional elements concerned in the determination of indi- 

 vidual life duration. It is now time that some attention be de- 

 voted to the environmental aspects of the picture. Here is where 

 the quest for personal longevity finds its greatest interest. While 

 in an academic or philosophical point of view it is doubtless de- 

 sirable to know as much as possible about the hereditary and 

 constitutional factors influencing life duration, still these are 



