418 



The Journal of Heredity 



iDcen fixed. Such a method is especial- 

 ly indicated where the characters are 

 of a kind determined so slightly by 

 heredity that genetic differences can be 

 recognized only on comparing lines 

 which have been kept distinct and free 

 from outside blood. This method, an 

 alternation of intensive inbreeding with 

 selection and crossbreeding of the few 

 successful lines must naturally be done 

 on a large scale and with the under- 

 taking of considerable risk. It is a 

 method adapted rather to experiment 

 stations than to private individuals. It 

 is. however, an important method and 

 has some parallel in the general history 

 of the breeds. Many of the early 

 breeders practiced close inbreeding. 

 Only a few like the Collings were 

 notably successful. The strains de- 

 veloped by these successful breeders 

 were intercrossed to found the present 

 pure breeds. 



For the individual breeder, however, 

 theory as well as practice indicate that 

 the most reliable method is the main- 

 tenance of a steady level in closeness 

 of breeding coupled with persistent se- 

 lection toward the desired type, the 

 requisite closeness of breeding depend- 

 ing, naturally, on the heterogeneity of 

 the foundation animals and the breed- 

 er's skill as a judge of livestock. 



Our analysis indicates that this was 

 the method pursued by Bates. In view 

 of Bates' success we may infer that the 

 degree of inbreeding practiced by him, 

 40 per cent, represents about the right 

 amount in the hands of an exception-* 

 ally able judge of cattle, working with 

 a material as heterogeneous as the 

 original Shorthorns. 



Summary 



It is shown that in establishing the 

 famous Duchess family of Shorthorns. 

 Thomas Bates started with Colling- 

 bred stock already about 40 per cent 

 inbred (i. e., which was 40 per cent 

 less heterozygous than the original 

 Shorthorns). During the eight genera- 

 tions which he bred himself, through 

 a period of about forty years, he main- 

 tained substantially the same level of 

 inbreeding by constantly introducing 

 just the right amount of fresh blood 

 to keep the percentage from rising 

 above 40 per cent. He used bulls, 

 whether of his own or other breeding, 

 which averaged about 40 per c^nt in- 

 bred. The relationship between the 

 Duchess cows and the bulls with which 

 they were mated, whether bred by 

 himself or others, was kept at such 

 point that a correlation of about + .60 

 would be present in purely hereditary 

 nondominant characters, throughout 

 the eight generations. Finally during 

 these eight generations a high correla- 

 tion, falling gradually from .76 to .57 

 was maintained with Colling's bull. 

 Favourite. 



It is suggested that these levels of 

 inbreeding and relationship yielded the 

 proper balance between the extreme 

 plasticity of the original heterogeneous 

 Shorthorn stock and the more com- 

 plete fixation of characters which 

 would have resulted from closer in- 

 breeding, to enable Bates, with his 

 great skill as a judge of cattle to main- 

 tain a high degree of vigor and to 

 mold a new type according to his 

 ideals on the basis of the type repre- 

 sented by Charles Colling's bull. 

 Favourite. 



Literature Cited 



^Darwin, Charles. Animals and Plants Under Domestication, 1868. 



^Wright, S. Mendelian Analysis of the Pure Breeds of Livestock. I. The Measure- 

 ment of Inbreeding and Relationship. Journal of Heredity 34:339-348. 1923. 



