CHAPTER XIII. 



Activity of Natural Selection at the Different Periods of Life Compared — Its 

 Influence on the Hereditary Transmission of Disease at these Periods — 

 Natural Selection Inoperative after the Procreative Period — Non-Trans- 

 mission of Evolutionary Gains after this Period — Average Limit of the 

 Procreative Period of Life in the Two Sexes. 



We have seen how natural selection tends, by an active 

 weeding-out process, , to prevent the hereditary transmission 

 of disease. It is evident that this process is inoperative after 

 the procreative period of life, for, if a disease then kills, the 

 race is in no way affected, the individual having already had 

 abundant opportunity to leave offspring, who will, if the disease 

 be due chiefly to peculiarity in S, tend to inherit it at the 

 same period of life as that at which the afflicted parent suffered 

 from it. 



Natural selection operates most actively before the procrea- 

 tive period, for, if during this period an individual be destroyed 

 by a disease due to faulty S, there is no possibility of his 

 leaving offspring to inherit the disease-tendency. Similarly, 

 it operates more rigorously during the early than in the later 

 part of this period, because, when the individual is cut off 

 in early procreative life, the possible progeny is less. In short, 

 natural selection is most vigorous before the procreative period, 

 inoperative after, and, during the period, severe in inverse pro- 

 portion to the number of possible children. 



We may, I think, safely conclude from the above that cases 

 of fatal disease due to inherited structural peculiarity are least 

 common before procreative life, that they gradually increase as 

 it advances, and suddenly increase after its limit has been 

 reached, not necessarily, however, becoming augmented year by 

 year, for the process of racial elimination is as inoperative 

 during the first as during the last year of post-procreative life. 



