CROSSES OF CAVIA CUTLERI. 19 



lefsen. This indicates that C. cutleri from Peru is the actual wild 

 ancestor of the guinea-pig or closely related to that ancestor. Since, 

 however, Nehring has reported that C. aperea (from Argentina) also 

 produces fertile hybrids with the guinea-pig, it seems likely that these 

 two species are closely related to each other and might interbreed freely 

 if their respective ranges were not completely separated. It seems 

 possible also that both species have contributed to the production of the 

 domesticated form, or that still other species have shared in producing 

 it. Further observations are needed to clear up this matter. 



It is evident that the mendelizing unit-character differences, which 

 distinguish one variety of domesticated guinea-pig from another, also 

 exist between guinea-pigs and the wild Cavia cutleri. They are inherited 

 in precisely the same way among the hybrids produced by crossing 

 guinea-pigs with C. cutleri as in crosses of one variety of guinea-pig with 

 another that is, they mendelize. It is evident that these variations 

 have arisen by a process of retrogressive or loss variation. For example, 

 in the matter of color varieties such as black, brown, yellow, and white, 

 which (in relation to the parent form) are known to breed true without 

 exception, it is evident that these have arisen by loss (or retrogressive 

 modification) of physiological processes which occur in the wild species, 

 since crosses with the wild form bring them back in a heterozygous 

 state, after which they continue to form all possible permutations and 

 recombinations with each other. Thus albinos of race C (which breed 

 true inter se and without crossing with some other variety could pro- 

 duce no other sort) if crossed with C. cutleri (which also breeds true) 

 produce in F 2 a definite series of color varieties. This series includes 

 all the color varieties of guinea-pigs more commonly known, such as (1) 

 golden agouti, (2) black, (3) cinnamon, (4) chocolate, (5) black-eyed 

 yellow, (6) brown-eyed yellow, and (7) albino. 



The mode of origin of the color varieties of guinea-pigs (and by 

 inference of other domesticated animals also) is therefore clear. These 

 varieties have originated by loss variations or loss "mutations." Is 

 this the means by which species themselves originate? Many biolo- 

 gists have recently advocated this view, as, for example, Lotsy, Baur, 

 and Bateson, but the present case affords rather strong evidence 

 against it. The color varieties of guinea-pigs differ from Cavia rufescens 

 and C. cutleri (undoubtedly distinct species) by the same mendelizing 

 color-factors, but there is no evidence that these two species differ from 

 each other by any color-factor. The two wild species are probably 

 distinct enough to show interspecific sterility, since one is known to 

 form sterile hybrids, the other fertile hybrids, in crosses with the 

 guinea-pig. Their specific distinctness accordingly can not be due to 

 such mendelizing factors as distinguish one domesticated variety from 

 another, but to something more fundamental in character, though less 

 striking in appearance. 



