CASTLE. — INBREEDING, CROSS-BREEDING, AND SELECTION. 769 



broods of fewer than 100 young are clearly like A broods, those of over 

 150 young are like M and N broods in size. Those between these limits 

 might fall in either category, but would be more characteristic of the A 

 series. 



Among the 89 broods reared from the children of cross-breds, 18 (or 

 20.2 per cent) are under 100 in size, while 30 broods (or 33.7 per cent) 



TABLE XX. 



Effect of selecting Pairs from Broods more or less Productive 

 but of Similar Ancestry. 



are under 150 in size. The expectation based on the hypothesis that 

 low productiveness is a Mendelian recessive character is that 25 per cent 

 of the broods will be low, which value lies between the two extremes 

 named above, 20.2 per cent and 33.7 per cent. 



Selection for High or Low Productiveness. 



The inheritance of different degrees of fertility is unmistakable through- 

 out these experiments and affords material for selection to act upon. 

 Whenever pairs are taken simultaneously from broods of the same auces- 

 vol. xli. — 49 



