178 Dynamic Evolution 



as men for example, are born to grow to 

 approximately equal size, and, consequently 

 to have an approximately equal number of 

 cells in their bodies. From these facts it 

 appears that a definite number of possible 

 divisions is the heritage of each animal, and 

 that when this number of divisions is ex- 

 hausted the animal dies. This failure does 

 not occur at all places at once as it did in 

 Holmes's One Horse Shay, but occurs some- 

 times in one place and sometimes in another, 

 and whenever it occurs in a vital organ, death 

 follows. For the purpose of our argument 

 , we may say that death occurs from old age 

 when the possible number of cell divisions is 

 nearly exhausted. 



Without attempting to justify the fore- 

 going hypothesis in regard to the possible 

 number of cell divisions, we may say that, if it 

 is approximately correct, there are two ways 

 in which longevity may be increased. 



First, by increasing the possible number of 

 cell divisions; and 



Second, by checking the rate at which cell 



