i9io.] 



Mendelism. 



28; 



having a black, it had a white face. It is clear, therefore, 

 that the something, whatever it may be, that causes the 

 blackness of the Suffolk is inherited independently of the 

 other characters, and can be replaced by the something which 

 produces whiteness, just as we can pick out one piece of a 

 mosaic and replace it by one of another colour without dis- 

 turbing the remainder of the picture. The method adopted 

 to secure this rearrangement of the mosaic is simply to inter- 

 breed the first crosses between individuals containing the 

 pieces we want, knowing that the offspring of the union, if 

 sufficiently numerous, will include the new combination we 

 are in search of. 



A question which will at once suggest itself to a practical 

 man is : Is the new individual pure ? Will its progeny not 

 occasionally revert to some of the ancestral characters which 

 it does not show ? Now, in the case of the white, hornless 

 ram, described above, we know that it will breed true because, 

 from observing the first crosses, we know that if the factor 

 that causes horns is present in a ram, the animal will have 

 horns, and if the blacking factor is present, the face is 

 speckled. In Mendelian phraseology, a recessive character is 

 always pure. But if a similar experiment had been carried 

 •out with cattle, the position of affairs would have been 

 reversed ; for, in the case of cattle, we have good reason to 

 believe that the hornless condition is dominant to the horned 

 condition, or, in ordinary language, the offspring of horned 

 cattle, whatever their immediate ancestry may have been, 

 will all be horned, whereas a polled animal may carry the 

 horned condition in its blood, and so produce horned off- 

 spring. It must be clearly understood that this quality of 

 dominance is accidental and not essential to the Mendelian 

 hypothesis, and that until actual experiment has been made 

 it is impossible to say whether it will manifest itself. Thus, 

 to return to the previous example, there is no dominance in 

 regard to face colour; if the white and black determinants 

 are both present, the face is speckled. In regard to wool 

 colour, however, we have reason to believe that white is 

 dominant to black, and that the appearance of black-woolled 

 sheep in a pure race of white sheep is due to the accidental 

 mating of two individuals each carrying a recessive black 



