48 BULLETIN 1121, U. S. DEPARTMENT OY AGEICULTUEE. 



most obvious in the fixation of such characteristics as color, number 

 of toes, and tendency toward the production of particular types of 

 abnormalities. There has also, however, been a significant differen- 

 tiation in the averages made in all elements of vigor. These ele- 

 ments of vigor have proved to be inherited independently of each 

 other. Each family has come to be characterized by a particu- 

 lar combination of traits, usually involving strength in some respects 

 with weakness in others. 



Crosses between different inbred families have resulted in a marked 

 improvement over both parental stocks in every respect, due allow- 

 ance having been made for the effects of size of litter on the other 

 characters. This improvement appears to its full extent in the 

 progeny of the first cross in the case of adult weight (about 12 per 

 cent) and resistance to tuberculosis (about 20 per cent). The mor- 

 tality between birth and weaning is found to depend about three- 

 fourths on the breeding of the young and one-fourth on that of the 

 dam. There is thus a marked improvement in the first cross (about 

 11 per cent) in spite of the inbred dam, but there is some additional 

 advance ia the progeny of a crossbred dam with an unrelated male. 

 In the rate of gain between birth and weaning, the breeding of the 

 dam and of the young are about equally important. An improve- 

 ment of about 16 per cent was obtained in this respect. Birth weight 

 depends largely on the dam — about three-fourths — and only one- 

 foTirth on the breeding of the young. There is thus only slight im- 

 provement before the second generation in which it amounted to 

 some 9 per cent. The mortality at birth is almost wholly a mater- 

 nal affair. Crossbreeding of the dam adds about 7 per cent to the 

 chances of the young. The heredity of the young also counts for 

 nothing in frequency or size of litter. The sire is somewhat more 

 responsible than the dam in the former case; the dam seems to be 

 wholly responsible in the latter. Frequency of litter was increased 

 over 30 per cent and size of litter over 10 per cent when both sire 

 and dam were crossbred. 



The number of young raised per year by an average mating depends 

 on four of the above elements of vigor — the mortality at birth, that 

 between birth and weaning, and the frequency and size of litters. 

 The relatively small improvement in crossbred matings in each 

 separate respect as given above, is compounded into an advance of 

 over 80 per cent in the combination, which goes well beyond the 

 superiority of the random-bred control stock over the inbreds. 



Analysis of the various crosses, indicates that the results are all 

 the direct or indirect consequence of the Mendelian m.echanism of 

 heredity. The fundamental efl^ect of inbreeding is the automatic 

 increase in homozygosis in all respects. An average decline in 

 vigor is the consequence of the observed fact that recessive factors, 

 more extensively brought into expression by an increase in homo- 



