INTRODUCTION. 



For several years I have been engaged in studies of heredity in 

 guinea-pigs. In the course of these studies all the common varieties 

 of guinea-pigs have been investigated by the method of experimental 

 breeding and something has been learned concerning their inter- 

 relationships and probable mode of origin. The actual origin of most 

 of these varieties is, however, unknown, as is true also concerning 

 most varieties of domesticated animals. One or two varieties have, 

 however, been made synthetically in the laboratory and it is conceivable 

 that, if we had the original wild stock to work with, from which the 

 domesticated guinea-pig has arisen, some or all of the existing varieties 

 might be synthesized anew and perhaps still others might be obtained, 

 and that in this way something might be learned of the method by 

 which new varieties arise. From considerations such as these I have 

 for several years been seeking to obtain living specimens of the wild 

 species which most closely resemble guinea-pigs. In 1903 I received 

 from Campinas, Brazil, 3 wild-caught individuals referred at the tune 

 to the species Cavia aperea, but since found to agree better with the 

 description of C. rufescens. From two of these animals young were 

 obtained, and crosses, the results of which have been described in 

 detail by Dr. Detlefsen (1914), were made with domesticated guinea- 

 pigs. It may be noted that all male FI hybrids were sterile, but that 

 the FI females were fertile, and that upon repeated crossing of these 

 with male guinea-pigs, a race of fertile hybrids was at last obtained, 

 these being, in the language of breeders, about guinea-pig, | rufescens. 

 From this result it seems doubtful whether C. rufescens has any close 

 genetic relationship to the domesticated guinea-pig, although by 

 hybridization it has been found possible to produce races (f or more 

 guinea-pig) which have derived certain characters from a rufescens 

 ancestor. 



Cavia aperea from Argentina has been crossed with the guinea-pig 

 by Nehring (1893, 1894) in Berlin, with the production of fully fertile 

 hybrids. This result indicates a closer relationship with the guinea-pig 

 than C. rufescens manifests. Darwin (1876), however, did not regard 

 aperea as the ancestor of the guinea-pig, because he found it to be 

 infested with a different species of louse. I have not myself been able 

 as yet to obtain specimens of C. aperea. Nehring (1889) has argued 

 with much plausibility that Cavia cutleri of Peru is more probably the 

 ancestor of the guinea-pig, for (1) it agrees closely with the guinea-pig 

 in cranial characters and it occurs in a region where guinea-pigs have 

 been for a long time kept in domestication, as is shown by the occurrence 

 of mummified guinea-pigs which had been buried with the dead. Natu- 

 rally I formed a strong desire to secure living specimens of C. cutleri for 



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