44 POPULAR SCIENCE MONTHLY. 



rise to the reproductive cells of the new individual. Not all reach this 

 fate; a certain number are apparently lost in their migrations, and 

 their late history could not be followed. So much seems certain — that 

 none of the cells which became segregated from the rest so early ever 

 became anything but reproductive cells, and that in no case do other 

 cells ever become reproductive cells. The shortest route observed be- 

 tween reproductive cells in the new individual and the egg from which 

 it developed did not exceed fifteen cell divisions. If the divisions had 

 been continuous at the rate of division frequently seen in fish eggs 

 all of them could have taken place in a day or two. Since the last gen- 

 eration of cells are really part of the new generation of individuals the 

 time between the beginning of development and the completion of pro- 

 visions for the next might have been but one or two days. 



Into the question of the origin of heredity and the hereditary 

 power of the reproductive cells I can not go at this time. Suffice it to 

 say that I consider the hereditary power of the reproductive cells the 

 result of a division of labor, just as the high contractile powers of the 

 muscle cells is the result of a division of labor.* 



It is evident from what has gone before that we can not say that the 

 individual became alive at any given point. Each individual is part 

 and parcel of many individuals who struggled and fought and aspired, 

 who lived, and, above all, succeeded. The fact that he is, is proof 

 positive that, as far as his ancestry is concerned, he deserves to be. He 

 has always been alive since the creation of his remotest ancestor. There 

 has never been a death in the direct line of his ancestry, and he forms 

 a link in a chain that is potentially endless and, in so far he is, through 

 his offspring, physically potentially immortal. 



* That the reproductive cells never develop into anything but reproductive 

 cells ought not to continue to confuse us, in the face of the fact that nerve cells, 

 for instance, never give rise to any other kind of cells and other cells are 

 never converted into nerve cells. Even in the regeneration of a lost arm only 

 those sorts of cells are regenerated which have representatives at the cut surface. 



