THE EVOLUTION OF SEX 29 



may be dispensed with. But this does not 

 of course contradict the view that the con- 

 jugation of unrelated forms may serve to 

 invigorate the constitution. 



From the recent investigations of Wood- 

 ruff at Yale, we select one experiment. 

 On May 1, 1907, he started with a " wild " 

 slipper-animalcule (Paramcecium aurelia) 

 which was isolated from an aquarium. When 

 it had divided into two, and each of these 

 into two, the four individuals were isolated 

 to form the starting-points of four pedigree- 

 lines. " The pedigreed culture has been 

 maintained by a specimen isolated from each 

 of these lines practically every day up to the 

 present time (May 1, 1912), thus precluding 

 the possibility of conjugation occurring and 

 facilitating an accurate record of the number 

 of generations attained." The varied culture- 

 medium was invariably boiled so as to render 

 the accidental introduction of " wild " indi- 

 viduals impossible. 



During the five years ending May 1, 1912, 

 3029 generations were observed, and those 

 at the end were as healthy as those at the 

 beginning ! There were 452 generations in 

 the first year, 690 in the second, 613 in the 

 third, 612 in the fourth, and 662 in the fifth. 

 The mean rate was over three divisions in 

 forty-eight hours. No periods of marked 

 physiological depression occurred the varia- 

 tions of rate that were observed being either 

 normal rhythms or the effects of environmental 

 changes of temperature and culture medium. 

 Woodruff concludes that " the protoplasm of 

 a single cell may be self-sufficient to reproduce 



