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CHAPTER XII 



THE UNIT-CHARACTERS OF RODENTS 



No group of mammals has been studied as thoroughly, in 

 respect to heritable characters, as have the rodents. This is 

 particularly true as regards those striking variations of the 

 coat which form the basis of the many recognized domestic 

 varieties. In nearly every case the distinctive features of 

 these several varieties are found to be Mendelian unit-char- 

 acters. As an example we may take the varieties of the 

 domestic cavy or guinea-pig, probably the first of the rodents 

 in point of time to be domesticated. Certainly in richness of 

 varieties it surpasses all others. It was domesticated by the 

 ancient Peruvians before the discovery of America and 

 formerly held an important economic place among the natives 

 of tropical America where it was reared as an article of food 

 in every cabin, a practice which to some extent still continues 

 among the poorer classes. Its variation in color and other 

 coat characters has been very extensive, unequalled in 

 amount perhaps among mammals other than dogs. Nearly 

 every distinct variety is characterized by the possession of 

 one or more Mendelian unit-character variations. At least 

 ten such unit-characters are concerned in the production of 

 these varieties. Several of these unit-characters have al- 

 ready been referred to. (See Table 12.) All but one of them 

 (" rough ") may be regarded as recessive unit-character va- 

 riations from the conditions found in wild cavies generally. 

 Perhaps the earliest in point of time, certainly the com- 

 monest among rodents wild or domesticated, is the albino 

 variation, in which the fur is white and the eye pink. This 

 makes its appearance as a sport, probably originally in a 

 single individual and later as a recurring variation among its 

 descendants. Albino individuals are undoubtedly at a dis- 

 advantage in the struggle for existence in a wild state because 

 of the conspicuousness of the albino to its enemies and also 



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