Longevity 177 



enough to cause it to increase about 200 

 per cent, in weight during that time, but the 

 decline in the rate of cell division is such that 

 during the second year he increases only about 

 20 per cent, in weight, during the fourth year 

 it is about 10 per cent., and by the time he is 

 twenty years old the rate of increase is re- 

 duced to about 2 per cent. 



This decline in the rate of cell division is 

 not the same for any two individuals, and 

 differs widely for different kinds of animals, 

 but in all cases in which the animal lives to 

 become adult there comes a time when the 

 rate of cell division just balances the wastage, 

 and growth in weight ceases. Still later there 

 comes a time when the rate of cell division 

 will no longer compensate for cell wastage, 

 and the animal shrinks in size and weight. 



It has been determined that, generally 

 speaking, cells are of comparatively uniform 

 size, and that the difference between a large 

 animal and a small animal is a difference in 

 the number of cells and not a difference in 

 their size. The different animals of any kind, 



