THE SIGNIFICAKCE OF FERTILIZATION 173 



hand, found that out of SO pararaecium individuals that had been con- 

 tinuously on the same food for many months in culture, only (3 per 

 cent, continued to live, and this low percentage was the same whether 

 the conjugating forms were of the same or of diverse ancestry. It may 

 be as Stevens ( '03) pointed out, that such low percentages were due to 

 the lowered vitalitv of the organisms in culture, but in all cases the 

 food medium was the same, and the explanation may lie in the fact 

 that the culture forms, having lived upon the same food material for 

 many months, were too similar to give rise to appropriate chemical 

 combinations upon fusing. The injurious effects of too close and too 

 prolonged inbreeding of higher forms may have their explanation in 

 such experiments, and similar experiments and observations on the 

 unicellular animals under culture may ultimately furnish the key to 

 the riddle of fertilization. 



