198 LEPORID^— ORYCTOLAGUS 



The 'Himalayan,' a white form, with symmetrical brownish-black 

 ears, nose, feet, and upper side of tail, was described as distinct by 

 Bartlett {Proc. Zool. Soc, London, 23rd June 1857, 159), under the 

 name Lepus nigripes ; it can, however, be produced by mating silver- 

 greys with the form called ' Chinchillas,' or light silver-greys (Bartlett, 

 Proc. cit., 1 861, 40). The study of these varieties has become a separate 

 branch of science, and they are dealt with in detail by fanciers and 

 Mendelians, whose works must be referred to for further information 

 (see Hurst's paper, mentioned above). 



Room must, however, be found for a summary of recent work on the 

 causation of yellow, black, and other varieties, a subject which has been 

 considerably elucidated by recent investigations (see Castle, Science, 

 25th January 1907, 151-153; 30th August 1907,287-291; 21st August 

 1908, 250-252; Hurst, op. cit). 



In a rabbit of normal, or, as Mendelians term it, " agouti " coloration, 

 the pigments of the hairs are laid down in rings as described above, due 

 to a definite cycle of activity in the hair follicles, which form the 

 pigments in a regular sequence. In instances of colour variation the 

 hair follicle may cease to form the pigments in sequence, with the result 

 that they are mixed, instead of being segregated, or, on the other hand, 

 one or more of them may be absent, or present in variable quantity, 

 resulting in various shades of pigmentation. 



The normal individual is apparently possessed of several factors : — 

 Ri, the factor for ringed hair ; Bl, the factor for black pigment ; the latter 

 possibly mixed with another factor, Br, that for brown or chocolate 

 pigment, which is known to be present in mice and cavies ; and Ye, the 

 factor for yellowish or reddish pigment. An individual lacking Ri is 

 black, the black colour being so abundant as to conceal the also present 

 yellow or brown. If a further factor Bl be lost, a yellowish or rufous race 

 results. Thus in uniformly coloured rabbits something is lacking which 

 is present in the normal individuals, and it is remarkable that varieties 

 lacking the factor Ri and possessing unbarred hair always lack also the 

 white colour of belly and under side of tail which is due to absence of 

 pigment from the terminal portions of hairs in these regions. Blue 

 or blue-grey rabbits are individuals possessing black pigment in a dilute 

 form, while the yellow apparently remains scanty in amount. 



White specimens are not, as regards colour, necessarily negative, 

 that is, entirely lacking the power of forming pigment, although some of 

 them may be so, and are then true albinos. Many white individuals 

 lack an aestivating or ferment-like substance, without which pigments 

 cannot be made visible ; but in other respects they carry all the colour 

 potentialities of pigmented individuals, which is shown by the results of 

 crosses between the two classes. White specimens may thus occur in 

 any of the above-mentioned categories. 



