Long'evity 191 



the fertile period has been extended to later 

 ages in correspondence with the extension 

 of life we know by observing animals which 

 live different lengths of time. The longer 

 is the normal life of the animals of any 

 given species, the later does the fertile 

 period begin. We may find an explana- 

 tion of this in the same thing which 

 explains increasing longevity. If late re- 

 production tends to check the rate of cell 

 division, so that the total number of cell 

 divisions involved in ordinary life are ex- 

 tended over a greater length of time, then 

 that checking of the rate of cell divisions will 

 cause a longer time to elapse before the 

 young animal has arrived at that stage of 

 development which will permit it to produce 

 germ cells. Briefly, that thing or action 

 which lengthens life lengthens all portions of 

 it, very much as all portions of a rubber 

 band are lengthened when the band is 

 stretched. 



Here we have set before us the process of 

 evolution from a lower to a higher stage. If 



