Theory of Rejuvenescence 23 



exist? Suppose that there were no death in man; there 

 would be no need for reproduction; and if reproduction 

 occurred it must soon result in overcrowding and in violent 

 killing off on even a greater scale than occurred in the 

 great war. And in these simple creatures the same over- 

 crowding must result, for reproduction every twenty-four 

 hours, without death, would in a few months pack all the 

 waters of the earth with a solid mass of these creatures. 



Reflecting thus on these things, many biologists came to 

 the conclusion that these creatures must get old and die 

 after all ; that otherwise they would not mate and reproduce. 

 We see indeed that not every individual gets old, for they 

 continue to live and reproduce for generation after genera- 

 tion. So it was thought that it must be the passage of 

 many generations that brings on age. The creatures, it 

 was held, begin young and strong; they divide again and 

 again, gradually getting old and weak ; it is only the indi- 

 viduals of these later generations that show the weakness of 

 age. 



Now, since in higher animals mating and reproduction 

 bring it about that an old and worn individual is replaced by 

 one young and unworn, so, it was reasoned, must mating in 

 these lower creatures cause an old individual to be replaced 

 by a new one. But when we observe the process, we do not 

 see a new individual replace an old one after mating; ap- 

 parently the old one continues to live and multiply as before. 

 To avoid this difficulty it was concluded that in these crea- 

 tures mating must rejuvenate the old individual; must in 

 some way get rid of the age and wear, leaving the same 

 individual, but physically young. 



This was the famous theory of rejuvenescence through 

 conjugation. It held that the young animal may live and 

 reproduce for many generations without mating, till thou- 



