216 Mr Doncaster, On the Inheritance of Goat-colour in Rats. 



usually has a colourless tip of varying length, and at least the 

 lower part of the fore limb is usually white. Type 6 (black 

 colour) is known in the fancy as " Irish " ; a good Irish has only 

 a small white patch on the breast. Types 3 and 5 have the head 

 and neck coloured, and the pigmented hairs may extend on to the 

 fore limb. There is almost always a coloured stripe along the 

 middle of the back from the hood to the tail ; the ventral surface, 

 sides, and limbs are white. The back stripe varies considerably ; 

 it may be broad, sometimes spreading out irregularly, it may be 

 narrow and even, and in other cases it is broken into a series of spots. 

 Very rarely it is quite absent. When the white patch of types 2 or 6 

 is very large, the animal approaches the darkest form of types 3 

 and 5, but only once have I had any hesitation in classing a rat as 

 belonging to one or the other. 



"Broken-marked" rats (i.e. irregularly piebald) do occur in 

 rare instances, but none have appeared in my experiments, nor 

 has there been any departure from the pigmentation described. 



It will be seen that pigment distribution in rats is much 

 simpler than in mice, since there are only two col our- types, and 

 two patterns in the coloured-and- white individuals ; the complica- 

 tions which arise in mice are therefore absent, and the scheme 

 of inheritance much simpler. From the practical point of view 

 this simplicity is counterbalanced by several disadvantages. Rats 

 breed much more slowly than mice, and take much longer to 

 reach maturity ; in my experience they do not often begin to 

 breed when younger than three months. The doe may take the buck 

 immediately after a litter is born, but commonly young appear at 

 intervals of six weeks or two months, often considerably longer. 

 It therefore takes a long time to rear a sufficient number of young 

 from any one pair. They are also very liable to disease, especially 

 of the lungs ; if this is to be avoided they require rather large 

 cages and frequent cleaning. Hence they take up a great amount 

 of space, and my work has been delayed owing to want of room in 

 which to rear many families simultaneously. The does do not 

 often eat or desert their young, though this happens occasionally, 

 but on the whole the infant mortality is very small. 



The most important work on colour-inheritance of rats is that 

 of Crampe (loc. cit). His papers were written long before the 

 re-discovery of Mendel's work, and are often very obscure, but the 

 following facts can be made out clearly from his results. Brown 

 (grey) is uniformly dominant over black, and both brown and 

 black over albino. The self-coloured varieties, and those possess- 

 ing small white areas on the lower surface (Nos. 1, 2, 6, 7) are 

 dominant over the coloured-and-white types (Nos. 3 and 5). All 

 seven varieties can probably be obtained pure by continued 

 selection, but this is much more easily effected with the more 



