FERTILITY 205 



view that hybridity or heterozygosity itself is the basis 

 for the observed vigor ; but they admit that another 

 interpretation is also possible. For instance, each of 

 the original strains may have been deficient in some of 

 the factors that go to make vigor. Together they give 

 a more vigorous individual than themselves. 



Whitney ran one line of hydatina through 384 par- 

 thenogenetic generations, when it died (Line A). An- 

 other line was carried through 503 generations, and at 

 the last report was in a very weakened condition (Line 

 B). When the former line was becoming extinct, he 

 tried inbreeding. From the fertilized eggs he ob- 

 tained a new parthenogenetic female. It showed 

 scarcely any improvement. The other line gave similar 

 results. In one case he again inbred for a second time. 

 He found that the rates of reproduction of lines A and 

 B were scarcely, if at all, improved. 



Whitney then crossed lines A and B. At once an 

 improvement was observed. The rate of reproduction 

 (vigor) was as great as that in a control line (reared 

 under the same conditions) that had not deteriorated. 



The experiments of A. F. Shull on hydatina were 

 somewhat different. He began with the twelfth gen- 

 eration from a sexual egg. The line was supposedly 

 not in a weakened condition. He inbred the line and 

 obtained from the fertilized egg a new parthenogenetic 

 series. After a few generations he inbred again. The 

 results are shown in the next table. It is clear that 

 there has been a steady decline despite sexual repro- 

 duction, measured by four of the five standards that 

 Shull applied, namely, size of family of parthenogenetic 

 females, and of sexual females, number of eggs per day, 



