OCR Text |
Show 82 MR. W. BATES0N ON COLOUR-HEREDITY [May 26, whites. The types are in fact definite, and cannot be built up by cumulative selection. I a m indebted tc Miss Douglas for much information as to the varieties of rats and for the loan of specimens. She tells m e that rats coloured otherwise than the varieties named are exceedingly rare. Irregularly piebald or spotted rats occasionally occur, but she has tried recently to obtain such rats from fanciers without success. In her experience the type 6 breeds true or nearly so. Of the blacks examined by m e this type had less of the brown hair than type 7. A striking feature appears from the rat-evidence, namely the absence of yellow, blue, chocolate, and indeed most of the varieties so familiar in fancy mice. On superficial examination, the colour of a wild rat is not very greatly different from that of a wild mouse. In rabbits also the yellow as well as the black forms are common. Yellows or yellow-ancl-whites are also familiar in guinea-pigs, fowls (buff, and " pile " ) , and pigeons. Miss Douglas has heard of a pair of cream-coloured rats, but otherwise I can find no records of any kind of yellow in the fancy. As blacks are so easily produced by resolution in the rat, the absence of the corresponding yellow and chocolate is remarkable. One is tempted to inquire whether the existence of black gametes does not suggest that yellow or at least chocolate gametes must also exist. The problem of their disappearance raises many important questions as to selective union between gametes, and others too elaborate to develop now. As there are no yellows, so also there are no chocolates. Another noteworthy fact is the complete absence of blue rats. This particular stage in the diminution of the amount of dark pigment is well known in mice, rabbits, cats, and several birds, but it is unknown in rats. There is of course no question that such forms would have been preserved if they had been seen by fanciers. Either yellow or blue rats would be worth several pounds. W e may take it therefore that these particular resolutions, or perhaps mutations, cannot be produced by any of the means by which they have been produced in other forms. Conceivably, if some distinct species were crossed with our fancy rats, some of these forms might be created. Similarly there are no " Himalayan " rats, i. e. pink-eyed with patches of colour (blackish or yellow), forms well known in rabbits, guinea-pigs, and in the "Japanese" waltzing mice. To sum up the evidence as to rats, we have clear proof of the segregation of certain types of gamete-the albino, the black-and-white, and the grey-and-white, though the ratios in which they are produced by heterozygotes are not yet determined. Further, there is proof that certain of the colour-types exhibit definite valency {Werthigkeit of Tschermak) and dominate over each other according to a regular system. Of the other colour-types one, viz., type 2, is almost certainly a definite heterozygote form, and is probably incapable of being made into a pure race. |