22 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



visible on the belly. Such animals show an extremely slight sprinkling 

 of agouti hairs when they become adult (figs. 6 to 9). 



(6) No tame agouti guinea-pig, to my knowledge, has ever shown a 

 ticked belly, by which term I understand a condition in which the 

 individual hairs are barred with yellow and have black tips and bases 

 (figs. 4 and 5). I do not mean that all wild C. rufescens individuals 

 and all wild hybrids are a very black agouti with ticked bellies. Such 

 is not the case. The agouti pattern in the wild, and in hybrids 

 receiving agouti from the wild, varies from a form very closely com- 

 parable to the tame to forms almost indistinguishable from black, the 

 latter occurring only in the hybrids. 



Modification of the Wild Agouti. 



To leave comparisons and return to the wild agouti pattern, it may 

 be said at the outset that we do not know how the different shades of 

 wild agouti are inherited when the wild C. rufescens individuals are 

 mated inter se. The wild were animals that would not bear much 

 handling, and so our records simply state that they were of the agouti 

 pattern, with some additional data such as ''dark" or "light." They 

 could not be classified as so many distinct forms, for their range was 

 great. However, it would have been desirable to know if the darker 

 forms were hj^postatic and whether any forms could have been gotten 

 which breed true to one shade as far as could be detected by our crude 

 methods of classifying by visual inspection. 



The apparent confusion and contradictions were only increased when 

 the wild were mated to non-agoutis to produce | wild hybrids, hetero- 

 zygous in agouti. Although these animals were heterozygous in the 

 agouti factor (each one having received its share of agouti from one 

 gamete, coming from the wild sire), they produced both dark and light 

 agoutis of various shades in addition to recessive non-agouti offspring. 

 All of the female wild hybrids were mated to non-agouti males up 

 through the matings of the J wild; hence we are sure of the source of 

 the agouti in every case, and no admixture of tame agouti could have 

 occurred. The \ wild females also produced both dark and light 

 agoutis, irrespective of whether the mother was dark or light. As the 

 wild agouti was being passed from one generation of hybrids to the next 

 more dilute generation of hybrids, one fact stood out very clearly. 

 Weaker agoutis gradually made their appearance; in fact, so weak was 

 the agouti becoming that it failed to restrict black altogether dorsally 

 and only very slightly on the belly in some cases (see figs. 6 to 9). 

 This weakening of the power of the agouti factor can not be attributed 

 to the fact that the wild agouti is alwaj^s less potent to restrict black, 

 which comes wholly or partly from the guinea-pig source; for, as has 

 been stated, some hybrid females with strong agouti produced young 

 with weak agouti, and vice versa. 



