6 A. FRANKLIN SHULL. 



rotifers of the latter experiments must have been more sensitive 

 to adverse conditions. 



With all these measures pointing in the same direction, the 

 evidence of decrease of vigor with successive inbreeding seems 

 conclusive. 



OTHER EVIDENCE RELATING TO VIGOR IN INBREEDING. 



In one of my earlier experiments (Shull, 1911), two partheno- 

 genetic lines of Hydatina were crossed and a new line started from 

 one of the FI fertilized eggs. The FI line was more vigorous, 

 as measured by size of family and rate of growth, than was either 

 parent line. This increased vigor in FI, which Whitney (1912) 

 has since shown to be general in Hydatina, is without doubt the 

 same phenomenon as the decrease of vigor with inbreeding. 

 Later experiments of my own (Shull, 1912), which involved 

 inbreeding twice in succession, though affording some evidence 

 of an attendant decrease of vigor, were in part contradictory. 

 In the light of the present evidence, these contradictions appear 

 to be merely the fluctuations, such as are found in Table I., 

 and all that was needed to clear them up was further inbreeding. 

 So far as Hydatina is concerned, there is thus entire agreement in 

 the results of different experiments and different investigators. 



In other animals and in plants there has been accumulated a 

 la-rge amount of information leading to the same conclusion, 

 though not without exception, that inbreeding is attended by 

 deterioration. On the animal side it has long been a common- 

 place among practical breeders that inbreeding, at least in many 

 cases, is followed by a weakening of stock. Results of scientific 

 value have been reported in recent years. Castle (1906) found 

 that inbreeding the fruit-fly Drosophila probably reduces produc- 

 tiveness slightly (though this reduction could be prevented by 

 selection). Moenkhaus (1911) inbred the same fly (Drosophila) 

 and found the operation attended by a considerable increase of 

 sterility (failure of the eggs to hatch). He was not inclined, how- 

 ever, to class sterility as a loss of vigor, since other attributes of 

 vigor were not perceptibly diminished. More decidedly favoring 

 the view that inbreeding reduces vigor is the older work of von 

 Guaita (1898) on the mouse and Ritzema Bos (1894) on rats. 



