A. FRANKLIN SHULL. 



one from a family in which only a few hatched were selected for 

 further breeding. From these two females were bred two 

 parthenogenetic lines, of four and ten generations; respectively. 

 In each line some females were paired with males of the same line, 

 and from one of the fertilized eggs of each lot a new partheno- 

 genetic line was reared. The members of these were again inbred, 

 and so on, six times in succession. There were thus obtained two 

 series of parthenogenetic lines, each one after the first obtained 

 by inbreeding from the line preceding. In one of these series, 

 numbered I. in the table, the numbers of families in the six lines 

 were 12, 4, 5, 17, 5, and 9, respectively; in the other series (II.) 

 the numbers of families in successive lines were 12, 10, 8, II, 21, 

 and 17, respectively. 



I shall attempt to show, in what follows, that there is a pro- 

 gressive decrease in the vigor of these six lines, from first to last. 



TABLE I. 



SHOWING DECREASE OF VIGOR, AS MEASURED BY VARIOUS CHARACTERS, IN Six 

 SUCCESSIVELY INBRED PARTHENOGENETIC LINES OF Hydatina senta. 



THE MEASURE OF VIGOR. 



Six distinct means of measuring the vigor of the several 

 .parthenogenetic lines are available. They are as follows: 



i. Size of family of parthenogenetic females. With few excep- 

 tions every daughter of a female used for breeding was isolated 

 and recorded. The average size of family was computed for 

 each parthenogenetic line separately. Families not completely 



