44 



Mr. II. Onslow. 



days a black precipitate was deposited. The ferment also reacted with 

 j>cresol, etc., and a dark grey colour was given when sufficient natural 

 chroraogen and peroxide had been extracted to yield the reaction without 

 the addition of tyrosine and hydrogen peroxide. The fluid from blue rabbit 

 skins — the dilute form of black — gave results similar in all respects, thus 

 confirming the supposition that the pigments producing blue and black are 

 identical. The yellow* rabbits employed carried the agouti factor, which 

 is indicated by their white bellies. These skins yielded an extract in 

 which no trace of coloration appeared, when either tyrosine or j>cresol 

 was added as a chromogen, even after four days' incubation. Care was, of 

 course, taken to remove the white bellies, in order to avoid the possible 

 effects of an inhibitor similar to that occurring in the bellies of agouti 

 rabbits (see p. 48). Many other chromogens were tried, such as tryptophane, 

 adrenalin, pyrocatechin, pyramidone, guaiacol and other polyphenols, but 

 no specific ferment could be detected. 



Similar results were obtained when orange* rabbits were used. In these 

 skins there is no fear of the presence of an inhibitor, as the bellies are 

 self-coloured. 



III. The Cause of Dominant Whiteness. 



It is well known that white animals and flowers may be divided into 

 two distinct classes — albinos or recessive whites and dominant whites. 

 Dominant whites when crossed with coloured varieties invariably throw 

 white offspring in the first generation, that is to say, they behave as a 

 dominant to coloured varieties. Such dominant whites exist among various 

 domestic animals, as for instance the White Leghorn fowl. The other type, 

 albino, behaves as a simple recessive when crossed with coloured varieties, 

 giving nothing but coloured offspring in the first generation. These albinos 

 occur among most kinds of domestic animals, and anomalously among wild 

 species. A further difference between the two forms lies in the fact that 

 the eyes of dominant whites are generally more or less heavily pigmented, 

 whereas the eyes of albinos are pink and almost entirely devoid of pigment. 

 For the sake of clearness, albinos will be called " recessive whites " through- 

 out the rest of this paper. Animals that are partially white or piebald 

 may also be placed under one or other of these categories. In these 

 animals the pattern, or, speaking more correctly, the white portions of it, 

 are usually recessive to colour, but in certain cases, such as the " English " 



* For the yellow rabbits I was indebted to Mr. J . Hammond, and for the orange 

 rabbits, which correspond in the chocolate series to yellow rabbits in the black series, 

 to Prof. E. C. Punnett, whose paper in the ' Journal of Genetics,' vol. 2, No. 3, p. 235 

 (November, 1912), gives a full account of this relationship. 



