ANIMAL BREEDING. II7 



to a conclusion, it has been found possible to explain the inheri- 

 tance of complicated characters by resolving them into a small 

 number of simple factors. 



The experiments with poultry have been mainly directed to 

 studying the inheritance of weight, colour, and hen-feathering 

 in the cock. The original cross in each case was between a 

 Gold-pencilled Hamburgh cock and a Silver Sebright bantam 

 hen. These two breeds differ greatly in size, and the study of 

 weight inheritance may first be dealt with. The Hamburgh is, 

 roughly, twice as heavy as the Silver Sebright. At the time 

 the work was started, practically nothing was known of the 

 inheritance of size, and probably many breeders, if challenged, 

 would have prophesied that the ultimate result would have been 

 the production of a strain of intermediate size — in other words, 

 that " blended inheritance " would result. In point of fact, 

 this was not the case. The first cross birds were of intermediate 

 size, but when a further generation of over 200 birds was raised 

 from them it was found that this consisted of all sorts of sizes, 

 ranging from birds smaller than the Sebright to birds larger 

 than the Hamburgh. Moreover, it was found that the small 

 birds bred true to size at once, and there is evidence that strains 

 of intermediate and of large size could also be established without 

 difficulty. The results support the view that there are definite 

 factors for size which are transmitted in the ordinary Mendelian 

 way — a point of considerable importance to the animal breeder. 



A second series of experiments was designed to test the 

 inheritance of " silver " as opposed to " gold." The results 

 have proved beyond question that colour in this case acts as a 

 single inheritable factor, and further that silver behaves as a 

 dominant to gold; in other words, that when a germ cell con- 

 taining the silver factor combines with a germ cell containing 

 gold, the silver is dominant to, or conceals, the gold, and the 

 resulting egg will hatch out to a " silver " bird. Further, this 

 experiment provides an illustration of an important principle of 

 heredity which is known as sex-linked inheritance. It has been 

 found in certain cases that well-marked characters in animals, 

 other than sexual characters, are linked up with the character 

 for sex; that is, that the factor for " maleness " in the germ 

 cell, for example, may be linked to one or more other factors, 

 which will therefore only appear in the male, and cannot by 

 any possibility appear in the female. Similarly the female may 

 exhibit certain characters which, in the germ cell, are linked to 

 the factor for " femaleness," so that they cannot be reproduced 

 in the male. There is evidence that high fecundity, or egg-laying 

 power, in poultry is linked up with the factor for " maleness," 



