22 



The Journal of Heredity 



and low ones produced daughters having 

 the same grade of egg productivity, 

 when mated to the same cock. If this 

 be the true result, it is obvious that 

 poultrymen should seek above all to 

 secure cocks from strains noted for their 

 high egg records and obtain hens from 

 these cocks for high egg production. 

 Pearl further attributes the fecundity of 

 the breeds of fowl studied by him to 

 three distinct Alcndclian factors, one 

 of which is sex-linked. The study of 

 sex-linked characters — that is char- 

 acters which are transmitted through 

 the male to half of his grandsons, or 

 through the female to her sons, etc., 

 is being carried on at the station along 

 many lines. 



INBREEDING. 



Considerable theoretical contributions 

 to the subject of inbreeding have been 

 made from this station, one of them 

 being a mathematical method of measur- 

 ing the intensity of inbreeding. The 

 practical breeder has been content to 

 say, in measuring the amount of in- 

 breeding of a descendant of the Short- 

 horn bull Ringmaster, let us suppose, 

 that a certain calf was "seven-eighths 

 Ringmaster blood." This statement, 

 however, does not give any definite 

 idea of the pedigree of the calf ; European 

 zootechnists therefore evolved the count 

 of so-called " free generations " which is 

 a good practical measurement and easily 

 understood. If, for example. Ring- 

 master a]:)i)eared three generations Ijack 

 in the calf's male line, and two genera- 

 tions back in the line through the dam, 

 there are two free generations on the 

 male side and one on the female ; adding 

 these together, the calf is said to have 

 three free generations, in respect to 

 Ringmaster. It has been found, to 

 carry the matter a step further, that 

 most winning English racehorses during 

 the last half century have had five free 

 generations; prior to that, four free 

 generations was the rule. When there 

 are no free generations, Thorouglibreds 

 are likely to be deficient in vigor (a 

 result commonly attributed to high 

 inbreeding), when there are more than 

 five free generations, they fail because 

 their speed (the j^roduct largely of 



inbreeding) has been swamped by out- 

 crossing, or lost by reversion. 



Pearl proposes to measure the degree 

 of inbreeding of any individual alge- 

 braically, by calculating the possible 

 number of ancestors in previous genera- 

 tions and comparing this nvmiber with 

 the number actually found in the 

 animal's pedigree. By the use of some 

 such accurate measurement as this 

 (which is described and illustrated in 

 bulletins 215 and 218 of the Maine 

 station) a further study of inbreeding 

 will be much more ])racticable, and 

 should throw light on one of the most 

 fundamental and debated problems of 

 practical genetics. 



That a certain amount of inbreeding 

 is advantageous in the maintenance of 

 blooded stock is now generally admitted ; 

 that an excessive amount is dangerous 

 is also pretty widely believed. Pearl, 

 like a great many other gcnetists, thinks 

 the question of degree, in itself, is of 

 secondary importance, the real question 

 being the quality of the animals inbred. 

 The Maine station has issued the follow- 

 ing counsel to breeders: 



' ' That a mating of such close relatives 

 [as brother and sister or parent and 

 offspring] will surely result in disaster 

 is one of the carefully nursed super- 

 stitions of breeding, which has often 

 been exploded but will doubtless always 

 be with us. It may be said that all the 

 evidence which may be gleaned from 

 the experience of stock breeders indicates 

 that the results which follow inbreeding 

 depend entirely on the nature of the 

 individuals bred. If one inbreeds weak 

 animals, lacking in constitutional vigor, 

 and carrying the detenninants of unde- 

 sirable qualities in their germ cells, the 

 ofifsi)ring resulting from such a mating 

 will undoubtedly be more nearly worth- 

 less than their parents. If, on the 

 other hand, one inbreeds in the same 

 way strong and vigorous animals, high 

 in vitality and carrying the genninal 

 determiners of dcsirai)le qualities, there 

 may be expected a corresponding in- 

 tensification of these qualities in the 

 ofFsjjring. The time has come when a 

 vigorous jjrotest should be made against 

 the indiscriminating condemnation of 

 inbreeding. It should be clearly recog- 



