EFFECTS OF INBREEDING AND CROSSBREEDING. 29 



fact that these experiments gave very much less increase than AC 

 confronts us with something of a puzzle. The difference is too great 

 to be dismissed as due to chance (5.8 XPE). The probable explana- 

 tion is a reciprocal physiological relation between frequency and size 

 of litter. It has, in fact, been found in the inbred stock that when 

 one litter follows immediately after another (i. e., in 9 or 10 weeks, 

 the gestation period), it is smaller than the average, and that a 

 large litter predisposes toward a delay before the appearance of the 

 next litter. (See Part I, Bulletin 1090.) These negative relations 

 were not very strong, and in the more vigorous random-bred stock 

 the relations were positive. We doubtless have, however, as sug- 

 gested in the case of the relations between mortality at birth and 

 mortality between birth and weaning, an unstable balance between 

 opposed influences. It was shown in Part I that the records of the 

 inbred and control stocks rose and fell in parallelism from year to 

 year in these two elements of fertility (as well as in all other elements 

 of vigor) . In the present paper it is shown that there is considerable 

 agreement between frequency and size of litter in the rise and fall 

 from season to season during 1916 to 1919 (Figs. 4 and 5). Thus 

 external conditions tend to produce a positive correlation. There 

 may also be common genetic factors which tend the same way. 

 The apparent conflict between the evidence from random-bred and 

 inbred stocks, referred to above, merely means that in the random- 

 bred stock the causes of positive correlation were not completely 

 balanced by the reciprocal physiological relation suggested above, 

 while it was overbalanced in the inbred stock. One would expect 

 to find the physiological relations more important in the weaker 

 inbred stock. 



In comparing AC with CC or Cl we have experiments in which the 

 inherent characters of the dam and young are essentially the same. 

 Owing, as it appears, to the influence of the crossbred sire, CC and Cl 

 produced litters distinctly more regularly than did AC. We have 

 here a situation in which a negative physiological correlation could 

 reveal itseK uncomplicated by any positive correlation. The high 

 record of AC relative to CC and Cl in size of litter and the opposite 

 relations in frequency of litter are the expected results on this 

 hypothesis. 



Similarly the relatively high record of CA in frequency of litter 

 may exert a slight depressing influence on its record in size of litter. 

 The argument that the sire exerts some influence on size of litter is 

 strengthened, but hardly enough to be relied upon. 



We may conclude that crossbreeding causes a marked increase in 

 the size of litters produced by females. Crossbreeding of the sire 

 may have some influence, but too slight to be demonstrated by the 

 present data. The heredity of the young appears to be wholly with- 

 out influence. It may be [added that selection of the dams (and 



