24 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



J wild, a single individual (9 140), though light-bellied, produced only- 

 young with ticked belly. 



Thus it is seen that light-bellied may produce the darkest shade and 

 vice versa. It may be objected that the difference between the lightest 

 and darkest is a small one, and renders close analysis and tabulation 

 impossible. Such an objection is hardly valid when one considers that 

 the darkest forms are often almost indistinguishable from black, whereas 

 the lightest form is almost as yellow as an ordinary golden-agouti 

 guinea-pig (figs. 4 and 5). Whether or not light agouti females would 

 gradually or quickly be replaced by dark ones upon continued crossing 

 with the guinea-pig can not be said, for further crossing of the hght- 

 bellied females was omitted at the time and no light-bellied females 

 occurred after the | wild generation, but a few light-bellied | wild are 

 still alive and, with these, it is hoped to investigate the question further. 



It is most perplexing to assign reasons for these various expressions 

 of the agouti factor. One can hardly suppose that the very darkest 

 agouti, which is almost black, possesses precisely the same thing which 

 was contributed by the wild. In some cases (9 75) the | wild was very 

 dark. In others (as through the series, ^991, |9723, t-Vc^1082) 

 the change was carefully watched and the transition was noted, but it 

 did not take place in one generation. It might be supposed that the 

 C. rufescens agouti factor has inherently less restricting power in the 

 hybrids than in its own species, but this explanation obviously will not 

 apply to those hybrids which are light-bellied, nor to those cases in 

 which a gradual loss of restricting power was observed to occur in a 

 series of generations. Furthermore, it does not explain why light 

 forms gave both light and dark, just as the dark forms gave dark and 

 light progeny. No matings of any description among tame guinea-pigs 

 have yet made it necessary to postulate a number of similar agouti 

 factors which are coupled. If wild agouti is held to be made up of Ai, 

 A2, Ag, . . . . A„, then it could be supposed that one or a number of 

 these factors dropped out and gave a weaker and darker agouti. This 

 would explain how 9 63, 9 68, 9 69, and other | wild animals happened 

 to be very dark, because of a weak agouti with less genes; but it would 

 never explain how some of the Fa offspring and all of the Fj offspring of 

 particular females could possibly acquire these lost genes again and 

 become light yeUow agoutis with an agouti factor that is more powerful 

 to restrict black. No admixture of tame agouti can be considered a 

 causal agency in the change, since tame agouti hybrids were not pro- 

 duced until the F4 generation. 



In analyzing the case, it must be remembered that the Cavia rufescens 

 agouti factor has been acting on Cavia rufescens black for centuries. 

 Whatever agouti is, it is something which determines physiologically a 

 rhythmical deposition of pigments in the growing hair. It is not sur- 

 prising that such an activator, or whatever it is that is contributed 



