EFFECTS OF INBREEDING AND CROSSBREEDING. 



59 



While the characteristics of a family probably reflected to a con- 

 siderable extent simply the average of the genetic factors in the orig- 

 inal pair, there was considerable evidence of segregation in the early 

 generations in characters associated with vigor, just as in the cases 

 of color, pattern, polydactylism, and tendency to produce monstrosi- 

 ties. In one family which was very weak in most respects in the early 

 generations, a single line of descent began to expand in the eighth 

 generation until it composed the entire family. This line revealed 

 itself as the most vigorous in nearly all respects in the entire stock of 

 inbred families. There were other less extreme cases of this sort. 



The study brought out a striking contrast between the effects 

 on vigor of hereditary and environmental factors. Favorable or 

 unfavorable conditions affect alike growth, mortality among the 

 young, and fertility, in all their aspects. On the other hand, hered- 

 itary factors which affect each character by itself appear to be much 

 more important than ones which affect general vigor. 



Finally the study illustrates what is one of the most important re- 

 sults of inbreeding, namely, the bringing to light and fixing of char- 

 acters in a family. In the case of color the mechanism of heredity 

 is thoroughly understood and the automatic fixation of type is a 

 necessary consequence of this mechanism. The similarity of the 

 phenomena in the other characters makes it probable that the mechan- 

 ism is essentially the same in all cases. 



APPENDIX. 



Table 7. — Data on the fertility of the different inbred families, 1906 to 1910. 



Fam- 



Size of litter. 



Num- 

 ber of 

 mating- 

 years. 



Num- 

 ber of 

 litters. 



Num- 

 ber of 

 young. 



Aver- 

 age 

 size of 

 litter. 



Litters 

 per 

 year. 



Young 



ily- 



1 



2 



3 



4 



5 



6 



7 



8 



year. 



1 



i. 



22 

 13 

 14 

 13 

 17 

 19 

 19 

 12 

 25 

 14 

 11 

 20 

 12 

 11 

 46 

 19 

 43 

 2 

 23 

 15 

 15 

 11 



13 

 40 

 41 

 47 

 21 

 45 

 71 

 41 

 28 

 52 

 31 

 11 

 35 

 25 

 14 

 65 

 30 

 109 

 20 

 43 

 35 

 31 

 16 



15 

 28 

 45 

 57 

 33 

 76 

 61 

 41 

 25 

 31 

 31 

 22 

 16 

 23 

 16 

 90 

 36 

 121 

 15 

 50 

 61 

 44 

 17 



3 



20 

 29 

 38 

 26 

 56 

 38 

 27 



8 

 19 

 18 

 10 



9 



9 

 11 

 45 

 28 

 63 



4 

 23 

 U 

 21 



1 



8 



11 



8 



21 



37 



9 



1 



14 



7 



5 



4 



2 



7 



22 



7 



28 



4 



10 



21 



9 



1 









10.9 

 37.8 

 35.0 

 40.9 

 25.6 

 56.2 

 56.8 

 33.4 

 18.9 

 35.7 

 24.9 

 14.4 

 20.6 

 16.6 

 14.0 

 60.9 

 34.4 

 86.3 

 10.7 

 38.6 

 40.0 

 34.9 

 11.6 



43 



119 



142 



172 



104 



221 



231 



140 



74 



145 



106 



61 



87 



71 



61 



275 



123 



367 



45 



154 



173 



122 



53 



99 

 313 

 423 

 518 

 316 

 701 

 711 

 397 

 180 

 393 

 306 

 179 

 212 

 177 

 178 

 779 

 353 

 1,034 

 123 

 431 

 554 

 350 

 133 



2.30 

 2.63 

 2.98 

 3.01 

 3.04 

 3.17 

 3.08 

 2.83 

 2.43 

 2.71 

 2.89 

 2.93 

 2.44 

 2.49 

 2.92 

 2.83 

 2.87 

 2.82 

 2.73 

 2.80 

 3.20 

 2.87 

 2.51 



3.94 

 3.15 

 4.06 

 4.21 

 4.06 

 3.93 

 4.07 

 4.19 

 3.92 

 4.06 

 4.26 

 4.24 

 4.22 

 4.28 

 4.36 

 4.52 

 3.58 

 4.25 

 4.21 

 3.99 

 4.33 

 3. 50 

 4.57 



9.08 



2 . 



2 

 5 

 3 

 3 



5 

 5 



1 





8.28 



3.... 



7.... 

 9 



1 

 2 





12.09 

 12.67 

 12.34 



11.... 

 13 



1 





12.47 

 12.52 



14.... 

 15 



2 





11.89 

 9.52 



17.... 



18 



3 

 5 

 

 3 



1 





11.01 

 12.29 



19.... 

 20 



1 



1 



12.43 

 in. 29 



21 





10.66 



23 



2 

 6 

 2 

 3 





12.71 



24.... 

 31.... 

 32 



1 | 



1 



12.79 

 10.26 

 11.98 



34 



1 



11.50 



35.... 

 36.... 

 38 



5 

 5 

 2 

 1 





11.17 



1 * 



13.85 

 10.03 



39.... 





11.47 



1 





