INFLUENCE OF INBREEDING ON IIVDATIXA SEXTA. 5 



Notwithstanding fluctuations that are usually small, but 

 occasionally large, the number of eggs laid per day is noticeably 

 less in the later lines than in the earlier ones. 



The number of days required to reach maturity remained prac- 

 tically unchanged throughout each series, though the temperature 

 was higher in the later experiments than in the earlier. The first 

 line was reared in November, the last line in June. The higher 

 temperature in May and June should have reduced the time 

 required to reach maturity in those months. In June of the 

 preceding year, two lines which were the ancestors of those given 

 in Table I., but had not been inbred, showed an average time of 

 1.42 days and 1.56 days, respectively, to reach maturity. The 

 fact that the rate of growth remained practically unchanged in 

 the inbred lines, notwithstanding the increase of temperature in 

 the later experiments, favors the conclusion that the later lines 

 were less vigorous. 



The proportion of cases in which the first daughter of a family 

 was not vigorous enough to become the parent of the next 

 generation shows so much fluctuation in the six lines separately 

 that I have combined them two by two. Although the irregu- 

 larities are not thereby completely removed, it is plain that the 

 later lines include a larger proportion of families bred from other 

 than the first daughter than do the earlier lines. 



In regard to the sixth measure of vigor, the difficulty of rearing, 

 figures are not available because records were not preserved. It 

 was evident at the time of the experiments, however, that the 

 difficulty of keeping suitable food cultures gradually increased as 

 the inbreeding proceeded Whereas each culture was satis- 

 factory for three or four days in the first experiments, they usually 

 lasted less than two days at the end. This was not due to 

 chemical changes hastened by higher temperatures in the later 

 cultures, for cool periods in the last experiments, when the 

 temperature was lower than the room temperature maintained 

 in the first experiments, did not make the cultures last as long 

 as in the earlier lines. Furthermore, that nothing was wrong 

 with the food cultures themselves was shown by rearing rotifers 

 from an entirely different source, presumably not often inbred, 

 and obtaining from them healthy and vigorous families. The 



