CASTLE. — INBREEDING, CROSS-BREEDING, AND SELECTION. 767 



ness may not reappear in the following generation. Hence we can only 

 say that low productiveness has a tendency to be inherited in alternative 

 fashion, skipping a generation, but the alternative character of the inher- 

 itance is not sharp as is that of albinism and other typical Mendelian 

 characters. 



If low productiveness were a typical Mendelian recessive character, 

 we should expect it regularly to reappear after skipping a generation, 

 and to occur on the average in one fourth of the daughters produced by 

 cross-bred pairs. But it does not always skip a generation. See Table 

 XIX. Further, when it does, as expected, skip the first cross-bred genera- 

 tion (F^ it sometimes fails to reappear in the following generation (F 2 ), 

 cross-breeding having apparently obliterated the low productiveness. 



Nevertheless, considering the fact that it is impossible to draw a sharp 



TABLE XVIII. 



AX Series. 

 (Cross 7, page 747.) 



d A 36.1 9 X Stock-jar $ 6 § 



3 PI I PI «* ii"! 



g LJ L J S>3 _S ° & 



.8 ; i r > S, w ^ 



A XI 33 132 22 62 43 



AX2 211 262 318 263 308 290 275 126 



line of division between high and low productiveness, the alternative 

 inheritance of the two conditions found respectively in race A and in 

 M or N is remarkably clear. 



If we class as low all complete broods of one hundred young or fewer, 

 and as high all broods of over one hundred young, then the various pairs 

 are grouped as shown in the left half of Table XIX. But if we class as 

 low all broods of less than 150 young, and as high all broods of over 

 150 young, the various pairs are grouped as shown in the right half of 

 Table XIX. 



The uncrossed N series in all its fourteen generations (Table IX) never 

 produced a complete brood containing as few as 150 young, and the un- 

 crossed M series (Table VIII) produced only two such broods, one of 67, 

 the other of 109 young. The A series during the period under considera- 

 tion only occasionally produced broods larger than 150. Accordingly 



