FUTURE OF THE SCIENCE OF BREEDING 169 



factors belonging to a given group, the American workers 

 have mapped each of the four chromosomes of Drosophila as 

 regards the position of the different factors, and the fact that 

 the results are on the whole self-consistent is further evidence 

 in support of their hypothesis. 



There are some difficulties in the way of this interpretation, 

 but to discuss them is beyond the scope of this essay. Never- 

 theless all biologists must feel that the work constitutes a great 

 step in advance. Many a barren discussion has taken place as 

 to the physical basis of heredity. Now at last, through the 

 application of genetic methods, we appear to be getting some 

 insight into the matter, and the suggested correlation between 

 the facts of breeding and cytological phenomena offers fair 

 hopes of substantial progress in the near future. The physicist 

 has been able to demonstrate the molecule ; it may be that 

 the biologist will soon be able to demonstrate the factor, that 

 penultimate entity upon which so many of the phenomena of 

 Ufe depend. 



Sex. — A line of genetic inquiry which has attracted many 

 workers is that of the experimental analysis of sex. The fact 

 that the two sexes are, among the higher animals, generally 

 produced in approximately equal numbers, at once suggests 

 that one of the sexes is recessive and the other heterozygous 

 for a factor upon which the distinction between male and 

 female depends. But which of the two was to be regarded as 

 containing the factor lacking in the other was a question 

 insoluble without further data. Fortunately such data soon 

 became available in the form of those peculiar cases called 

 sex-linked. 



Many breeds of poultry possess both gold and silver 

 varieties, which, as a rule, differ only in the ground colour of 

 the plumage being golden orange or white. Gold and silver 

 pencilled Hamburghs are a good instance, while the little 

 gold and silver laced Sebright bantams are another. Experi- 

 ments have shown that silver behaves as a dominant to gold, 

 but there is a remarkable peculiarity in the results obtained 

 from reciprocal crosses (Fig. 7). When a silver cock from a 

 pui'e silver strain is crossed with a gold hen, all the chickens 



