PART II. THE EVOLUTION OF SEX 



CHAPTER VII 



Limitation of the powers of cell-division Rejuvenescence by conjugation 

 of gametes The origin of sex in the Protista. 



BY the process of cell-division an unbroken continuity has 

 been established in the chain of living things from the earliest 

 appearance of unicellular organisms to the present day. Every 

 cell is the descendant of pre-existing cells and, in accordance 

 with the theory of evolution, all cells which exist to-day, distri- 

 buted amongst the bodies of countless millions of different 

 organisms, could, if our knowledge were sufficiently complete, be 

 traced back to a single ancestral cell. 



It by no means follows from these considerations, however, 

 that there is, under natural conditions, no. limit to the ordinary 

 process of cell-division. On the contrary it is well known that 

 in any cell family, whether belonging to a unicellular or a multi- 

 cellular organism, the power of multiplication tends to become 

 exhausted, and, if that particular cell family is to continue its 

 existence, has to be in some way renewed. 



Take, for example, an ordinary ciliate or flagellate Protozoon, 

 which multiplies by simple fission. If a single individual be 

 isolated and placed in water containing suitable food material, 

 and kept under suitable conditions of temperature, light and so 

 forth, it will multiply very rapidly, until possibly hundreds of 

 generations of separate cells have been produced and the total 

 number increased perhaps to millions. But under ordinary 

 circumstances a time presently arrives when the individuals 

 begin to show signs of exhaustion, accompanied by physical 

 degeneration, and to slack off in their rate of multiplication. 

 They may be stimulated to renewed activity for a time by special 

 feeding or by constantly varying the culture medium, 1 but in a 



1 Mr. L. L. Woodruff has kept a culture of Paramoecium under observation for 

 nearly three and a half years, taking precautions to prevent the possibility of con- 

 jugation, but constantly varying the culture medium. During this time more than 

 two thousand generations of Paramoecium were produced by repeated fission an 

 B. G 



