llß Hagcdoorn. 



necessary to assume that every wild species of rats possesses X, Y 

 and Z. A second species may possess V and W, and have a colour 

 similar to that of the first species. If now the two species are crossed, 

 the hybrids are heterozygous for two or three of these genes. F^ 

 animals, which do not possess Z need not l)e albinos, they may have W, 

 which has a similar influen<^e upon the chain of processes leading to 

 complete pigmentation. 



Fo animals without W may have Z, and need not be albinos, but 

 those who lack both Z and W will be albinos, possessing a new 

 recessive cliaracter. A cross between two similar species may in this 

 way produce recessive novelties, the novelty originating in a ratio of 

 one in 16 or 64. Albinos, when compared to either pjirent species 

 will be seen to lack only one gene, W if compared to the parent- 

 species without it, and Z if mated to the parentspecies having W, but 

 lacking Z. 



It may be however that we have a species possessing both Z 

 and W. Mated to this, our albino would behave as a double recessive. 



Roan. 



We saw that three novelties, sports, of aberrant colour were caught 

 on the grounds of one sugarplantation Ketangoengan, two creamyellow 

 animals, two paleagouti and one albino. A few weeks previous to the 

 arrival of these animals we received one adult female from the same 

 plantation, which was spotted all over with small spots of a roan coloui-. 

 This female gradually became lighter and moulted out into a light silver 

 roan. She stayed in Buiteuzorg and did not breed while we had these 

 rats. In Paris, Prof. Dujardin Beaumetz of the Institut Pasteur bred some 

 Mus rattus, and gave us a few pairs, aniong which was a male with 

 the same character, a light silver, the colour of the common black- 

 agouti Mus rattus of France, but with a coat heavily interspersed with 

 white hairs. 



Origin of the wild sports. 



What can have been the origin of all these novel colours among 

 the rats of Ketangoengan? A few years ago the answer would 

 undoubtedly have been „lossmutation". It is not so long, since the 

 role of crossing as a cause of such variations is rightly understood. 

 When the Geneticians were thinking of genes as of determinants for 

 characters, which each called into being a special organ or (juality, it 



