FERTILITY 199 



a mixed diet — on the kind of materials that it would 

 be likely to meet with in nature, alternating with hay 

 and other infusions. He found no degeneration, and 

 at his last report his still vigorous strain was in the 

 3000th generation. 



Recently Woodruff has found in his long-lived race 

 that under proper conditions individuals will conjugate. 

 Woodruff and Erdmann found in this race, although 

 not allowed to conjugate, that periodically the macro- 

 nucleus breaks down and several micromere divisions 

 take place. Finally a new nuclear apparatus of micro- 

 nuclear origin is reconstructed. The process is com- 

 parable to the nuclear changes prior to conjugation 

 except that the last micronuclear division is omitted. 

 This periodic change is not peculiar to this race of 

 Paramoecium, but appears in other races that regularly 

 conjugate. 



Let us turn now to the other side of the question 

 and see what results cross-fertilization has given. 



Hyde has found that if two strains of flies with low 

 fertility are crossed, there is a sudden increase in the 

 output, as seen in the diagram (Fig. 96). The facts 

 show clearly an improvement. More eggs of each 

 strain are fertilized by sperm from the other strain 

 than when the eggs are fertilized by sperm from the 

 same strain.^ In this case the results are not due 

 to a more fertile individual being produced (although 

 this may be true) but to foreign sperm, acting better 

 than the strain's own sperm. The evidence, as such, 



^ The upper line Fi-Fis gives the average output of flies per pair. 

 Below this line the percentages mean the number of isolated eggs 

 that hatched. 



