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. 



How can we harmonize these different results ? 

 It is hazardous, perhaps, to offer even suggestions, but 

 if we assume that in a medium not properly balanced 

 paramcecium is likely to degenerate in the sense that it 

 loses some of its hereditary factors, we can understand the 

 failure to become normal when this has once taken place 

 even in a new environment. Temporarily the decrepit 

 individual may be benefited by a change, but not per- 

 manently if its hereditary mechanism is affected. In 

 Woodruff's experiment the normal environment brings 

 about no degenerative changes in the hereditary mech- 

 anism and the race continues to propagate indefinitely. 



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, 

 does not show whether this is due to each strain having 

 degenerated in certain directions, or to some other 

 kind of a change in the heredity complex. 



