232 GENERAL BIOLOGY 



further unlooked-for result was obtained. As was 

 expected, there appeared in the F 2 generation the 

 three forms, walnut, rose, and pea. But there also 

 appeared a definite proportion of single-combed 

 birds, and among many hundreds of chickens bred 

 in this way the proportions in which the four forms, 

 walnut, rose, pea, and single, appeared was 9:3:3:1. 

 Now this, as Mendel showed, is the ratio found in an 

 F 2 generation when the original parents differ in 

 two pairs of alternative characters, and from the 

 proportions in which the different forms of comb occur 

 we must infer that the walnut contains both domi- 

 nants, the rose and the pea one dominant each, while 

 the single is pure for both recessive characters. This 

 accorded with subsequent breeding experiments, 

 for the singles bred perfectly true as soon as they 

 had once made their appearance." 



The " Mendelians " have devised an ingenious 

 hypothesis which explains very plausibly the findings 

 in the above experiment, but to carry the details 

 further would take us too far afield. The single 

 comb, however, that appeared in the experiment 

 just quoted is the sort of a comb possessed by the wild 

 jungle-cock, which is considered to be the ancestor 

 of all our domestic breeds. Its unexpected ap- 

 pearance after perhaps thousands of generations 

 of fowls in which it was absent, that is, replaced 

 by another variety of comb, is spoken of as a re- 

 version or throw-back. This puzzling phenomenon 

 is familiar to all animal and plant breeders. With 

 the light which the studies of Mendelian inheritance 



