THE CAUSATION OF DISEASE. 269 



instance, the men of twenty successive generations lead 

 physically healthy lives till fifty-three years of age, then any 

 individuals whom they beget at this period will reap the full 

 advantage of ten years spent by the father in healthful physical 

 life. Under existing circumstances, however, the race is largely 

 deprived of this advantage, and it is needless to say that, as 

 far as the hereditary principle is concerned, old age can have 

 no influence on it, physically. 



In short, the limit of procreative life, so far as I can see, 

 practically determines the limit of evolution as regards age. 

 Unless the procreative period be extended in women, we can 

 never expect evolution in her to proceed beyond the present 

 average climacteric period, but it might be pushed on to a much 

 further limit in man. This conclusion follows from (1) the 

 principle of heredity as limited by sex ; (2) the principle of 

 heredity at corresponding ages. 



It is much easier to study the mental evolution of the 

 human race than its physical evolution, for mental evolution 

 leaves monuments as lasting as they are various. That the 

 rate of evolution is different for the two sexes there can be 

 no doubt, the female sex evolving more rapidly, both in mind 

 and body, than the male.* 



But whether the evolution goes on much longer in the life 

 of the man than of the woman, I have not been able to satisfy 

 myself. I have before me a series of notes which were taken 

 with a view to estimate the rate and limit of mental evolution 

 in the woman, and most of them tend to show the period 



* The earlier occurrence of puberty in the female results, I suggest, from 

 the fact that the sexual struggle has chiefly been among the males for the 

 females. For the sake of argument, let us suppose that puberty occurred 

 originally at the same age in both sexes. Directly the female attained sexual 

 life she would be eagerly sought after ; the younger males would, however, 

 have very little chance in the struggle against older and more vigorous males, 

 so that the tendency would be for the male sexual system to remain idle 

 until some time after puberty ; and, supposing this to occur for thousands of 

 generations, it would follow from the principle of heredity at corresponding 

 ages, and from what we know of the effects of use and disuse, that the onset 

 of puberty would tend to be postponed in the male. If this explanation be 

 correct, we may expect the period of puberty to gradually recede in the 

 female, owing to the fact that the woman— at all events, in civilized countries 

 — seldom marries until some years after puberty. 



