320 CARNEGIE INSTITUTION OF WASHINGTON. 



those experiments. It is hoped that his results may shortly be ready 

 for publication in full. A preliminary paper has already appeared 

 (see Bibliography, p. 49) . His results are concerned principally with 

 a study of interspecific sterility and its gradual replacement with 

 complete fertility in later generations of the hybrids between the 

 guinea-pig and Cavia rufescens. 



Among the more recently started lines of experiment which are 

 now in progress and are occupying the greater part of my attention 

 should be mentioned those with guinea-pigs obtained in Peru two years 

 ago. The small wild species obtained at Arequipa breeds readily in 

 captivity throughout the year, producing litters of from 1 to 3 young. 

 Two generations reared in the laboratory show no deviation from 

 the typical form of the wild ancestors. When this species is crossed 

 with the domesticated guinea-pig, which is about twice its size, 

 hybrids are obtained which are of about the size of guinea-pigs but 

 show the color of the wild species, with intensification of the colors 

 in most cases, and invariablj^ so when an intensely colored race of 

 guinea-pig is used in making the cross. The next generation of 

 hybrids (Fj) produces a great number of color varieties, in full 

 accord with the Mendelian expectation. It is too early to say what 

 the adult size of the hybrids in this generation will be. The complete 

 fertility of the Fj hybrids is noteworthy as indicating no specific dis- 

 tinctness between the wild form and the guinea-pig. This is in 

 marked contrast to the behavior of the hybrids with Cavia rufescens 

 already referred to. It supports the idea that the wild species still 

 occurring among the Andes is the form from which the domesticated 

 guinea-pig is descended. 



A domesticated and a (possibly) feral stock of guinea-pigs, which 

 were obtained in Peru two years ago, have yielded two series of novel 

 color varieties entirely unknown hitherto, so far as I can learn, in North 

 America and Europe. A preliminary description of these will shortly 

 be published. My colleague, Mr. C. T. Brues, of the department of 

 entomology of the Bussey Institution, recently returned from Peru, 

 whither he went to study insect carriers of tropical diseases. He 

 has kindly brought back 8 guinea-pigs from a third locality in Peru, 

 among which occurs one of the variations which had cropped out in 

 the stocks obtained in 1911. To study more intensively these novel 

 variations in guinea-pigs and also certain problems of inheritance in 

 rabbits which have recently taken on new aspects, we are enlarging 

 our accommodations for small mammals and equipping them with a 

 hot-water heating apparatus, to reduce as much as possible the losses 

 of animals in cold weather. The expense of these changes will be 

 borne by the Bussey Institution. 



The particular objects which the writer has in mind to accomplish 

 in future work are (1) a further study of heredity in the guinea-pig 



