340 



The Journal of Heredity 



clearly to the eye their features of 

 superiority. 



Bakewell's example was followed 

 with enthusiasm by other breeders and 

 closely bred strains of ths best local 

 tvi:)es of animals began to appear in all 

 parts of England. The Colling broth- 

 ers began following this system with 

 the shorthorned cattle of the north of 

 England. Their work was carried on 

 by other breeders, notably by Thomas 

 Booth and his descendants and by 

 Thomas Bates, with whose methods 

 we are to deal in a succeeding paper. 



While many of these early breeders 

 had notable success with close breed- 

 ing, injurious effects seem to have 

 been encountered with such increasing- 

 frequency in later years as to discour- 

 age the practice. The superior types 

 which had been developed in each re- 

 gion, were, however, maintained and 

 developed into the pure breeds as we 

 know them. Herd and flock books 

 were established for the recording 

 of pedigrees. Efforts at improve- 

 ment took the form of selection 

 within these pure breeds and in 

 the grading up of common stock by 

 the continued use of purebred sires. 

 This is essentially the status of the art 

 of livestock breeding today. 



During the last quarter of a century 

 a real science of breeding has been de- 

 veloping about the principles of 

 heredity discovered by Mendel. Men- 

 delian heredity has been found to be 

 the regular mode of inheritance for 

 all sorts of characters and in all sorts 

 of organisms. We have come to be- 

 lieve that it is the general law of hered- 

 ity under sexual re])roduction. 



With accurate knowledge of the 

 principles of heredity, the hereditary 

 characteristics of many plants and of 

 the small and ra]jidly breeding labora- 

 tory animals can be controlled with 

 something of the precision of the chem- 

 ist working with non-living materials. 

 Livestock breed relatively slowly, how- 

 ever, and the large generations obtain- 

 able from laboratory animals are not 

 feasible. Genetic analysis must thus 

 be slow. Nevertheless, genetics has an 



important contribution to practical 

 breeding in the insight which it gives 

 into the results of the long-known 

 mass methods of breeding : assortative 

 and disassortative mating, selection 

 and culling, inbreeding and outcrossing. 

 This is particularly true in the case of 

 inbreeding and outcrossing. 



The Effects of Inbreeding and 

 Crossbreeding 



The principal effect of inbreeding, 

 we find, is in automatically making 

 homozygous some combination of the 

 factors which were heterozygous in the 

 original random-bred stock. Imme- 

 diately related to this is the increase 

 in uniformity in an inbred stock, which 

 makes it possible to recognize genetic 

 differences which would otherwise be 

 overlooked. Increased prepotency in 

 outside crosses is another direct con- 

 sequence of increase in homozygosis. 

 A usual but not necessary decline in 

 vigor in all respects is explained as due 

 to a tendency for recessive factors, 

 brought to light by increased homozy- 

 gosis, to be more frequently deleterious 

 than dominant factors. This tendency 

 in turn is explained as due merely to 

 the greater rapidity with which natural 

 selection can eliminate the deleterious 

 dominant variations that arise in a 

 random-bred stock. These conclusions 

 and their interpretations have been test- 

 ed with diverse kinds of plants and 

 animals. Among mammals. Miss King's 

 experiments^ at the Wistar Institute 

 with inbreeding and selection in a line 

 of rats have shown clearly that in- 

 breeding does not necessarily lead to 

 deterioration. The experiments of the 

 Bureau of Animal Industry'' with 

 twenty-three different inbred lines of 

 guinea pigs have also demonstrated 

 that even twenty-five generations of 

 brother-sister mating may not cause 

 any obvious degeneration. They have, 

 however, demonstrated that some de- 

 cline with inbreeding is the usual result 

 in such characters as weight, fertility 

 and vitality. They have also brought 

 out a conspicuous differentiation among 



