246 AGE, GROWTH, AND DEATH 



recognition and from that of our friends, because we have acquired 

 certain habits of activity which we are able to keep up, but we 

 are not able without ever-increasing difficulty to turn to new 

 forms of mental activity, or in other words, to learn new things. 

 When we grow old we may still continue to do well the kind of 

 thing which we have learned to do, whether it be paying out bills 

 at a bank or paying out a particular set of scientific ideas to a 

 class of students. If we try to overstep the limits of our acquired 

 expertness we find that we are held up by this sense of permanent 

 mental fatigue. Usually this condition comes about gradually, 

 but I have known, as I presume you all have, several cases in 

 which it has appeared suddenly, where a man who up to a certain 

 time was fond of mental exertion suddenly ceased to be mentally 

 active. We have probable illustrations of this in the careers of 

 well-known scientific men. I think the theory of permanent 

 mental fatigue, in connection with the theory of gradual decline 

 which we are considering this evening, could be usefully devel- 

 oped and might well be utilised by the psychologists in their 

 studies." 



As in every study of biological facts, there is in the 

 study of senescent mental stability the principle of vari- 

 ation to be kept in mind. Men are not alike. The 

 great majority of men lose the power of learning, 

 doubtless some more and some less, we will say, at 

 twenty-five years. Few men after twenty-five are able 

 to learn much. They who cannot, become day-labour- 

 ers, mechanics, clerks of a mechanical order. Others 

 probably can go on somewhat longer, and obtain higher 

 positions; and there are men who, with extreme varia* 

 tions in endowment, preserve the power of active and 

 original thought far on into life. These of course are 

 the exceptional men, the great men. 



