Data for the Problem of Ecolution in Mn. 171 



hack upon measuring the effect in man, and thus obtaining what may 

 well be considered as a minimum value of the influence under dis- 

 cussion. 



In the course of our investigations we have seen that the relationship 

 between fertility and duration of life does not cease with the fecund 

 period. We thus reach the important result that characters which 

 build up a constitution fittest to survive are also characters which 

 encourage its fertility. This result is of great value from the standpoint 

 of the differentiation of type, where it is absolutely necessary that the 

 fittest to survive should also be the most fertile.* On the other hand, 

 we note that duration of life is a character capable of modification by 

 reproductive selection, and we suggest that a considerable part of the 

 increased expectation of life observed in recent years may be due to 

 this cause. In the case of the American statistics, we see at once how 

 it can replace a remarkably short-lived stock by a longer-lived stock, the 

 bulk of the offspring coming from the longer-lived members. 



* ' The Grammar of Science,' second edition, pp. 448 



