44 



SIZE INHERITANCE IN RABBITS. 



namely, 10.43 and 4.24. It is fair to compare their coefficient taken 

 together (8.75) with other fraternities consisting of more than one litter. 



Tabi.k 8. — Coefficients of variability in weight of the several families of rabbits studied. 



Family 

 No. 



647. 



1471. 

 1491. 



1493. 



1531. 



1532. 

 1537. 



2037. 



Coefficient 



variability 



inFi. 



5.16 



4.09 

 6 22 



8.75 



Number 

 of Fi indi- 

 viduals. 



vSabilS N-^ ' 

 in back™*., £5&5l"E 

 with father, '"^.duals 



10.52 

 5.88 

 10.27 

 13.62' 

 10.46 

 6.32 

 13.56 

 8 30 

 9.29 

 9.94 

 13.46 

 10 45 

 10.27 

 7.35 

 8.23 

 10.83 



7 



7* 

 10 



7 

 8 

 8 



12 

 fi 

 lfi 

 14 

 H) 

 10 



a 



23 



*In this case the back-cross was with the mother. 



SUMMARY. 

 The weights, then, show a tendency for back-cross fraternities to be 

 more variable than Fi fraternities, although five back-cross fraternities 

 in 16 showed no greater variability than some Fi fraternities. Although 

 not exact, the weights indicate the same fact that was shown by the 

 bone measurements — namely, that there is a greater variability in the 

 second than in the first generation of hybrids. 



CONCLUSIONS. 

 EXPERIMENTAL. 



The conclusion to be drawn from all these observations is clear. 

 Whether on the basis of the comparison of the ranges of litters of the 

 first filial generation with those of their corresponding back-crosses, 

 or whether on the basis of the relative variability of the two generations, 

 as shown by the standard deviations of the coefficients of size, or by the 

 distribution of frequencies in relation to the parents or grandparents, 

 or, finally, by their body weights, there is found a consistently greater 

 diversity of sizes in the back-cross than in the first-hybrid generation. 

 Characters occur among the second generation that are smaller than 

 the corresponding characters in the small parent; others that are above 

 the modes of the first generation large parents. 



This difference in the two generations appears in spite of a fact that 

 might be expected to hide it. Several of the original mothers were 

 undoubtedly partially heterozygous in size, as is shown by their pedi- 

 grees (table 3). This would lead one to expect a greater variability in 

 their offspring from crosses with the small male than was shown by 



