310 THE GRAY RAT. 



If hunger happens to come upon the rat popula- 

 tion, the feebler members of the community fall 

 to the stronger which means that the very young 

 are killed by the old, and that the very old are 

 killed by the middle-aged. In short, only the 

 fittest, which probably are the middle-aged, escape 

 death at the hands of their own kind. 



It would seem that gray rats habitually kill off 

 the old male members of their communities ; and 

 it may be observed that when a rat is found living 

 alone, as, for example, the summer rat described 

 as living in a bank-burrow by the river Wharfe, 

 it is invariably an old buck which, having had one 

 attempt made upon his life by his clansmen, has 

 sense enough to avoid further encounters by living 

 a life of isolation and solitude. 



LENGTH OF LIFE. 



Sixty-eight years may be taken as the average 

 length of man's life, which is approximately four 

 times the period required for him to arrive at full 

 organic, if not muscular, development. This rule, 

 however, can seldom be applied successfully to 

 animals of the lower order. By their rate of living 

 man is a short-lived creature. Few of them have 

 reached the zenith of their powers by the period 

 at which, correspondingly, senile decline begins to 

 show in human beings. This, together with such 

 data as are given elsewhere in this book, seems to 

 indicate that it is the mind, rather than the body, 

 which decides a creature's length of life. Man 

 wears badly in the carnal order of things, because 

 his mind is more active than his body ; while as 

 a general rule creatures that hibernate, and whose 

 minds, therefore, are inactive for a portion of their 

 existence, outlive those that are astir the year 



