GENETIC STUDIES ON A CAVY SPECIES CROSS. 



GENERAL INTRODUCTION. 



The genetic studies herewith presented were made possible for the 

 author, by the reception of the foundation stock, in December 1909, 

 from Dr. W. E. Castle. The lirst crosses had been made in 1903, and 

 about 200 of the wild and intense wild-blooded hybrid animals had 

 been born w^hen the stock was received. The birth records, the weights, 

 and such skeletons as had been saved, as well as the living hybrids, 

 were made available to the author, v/ho here expresses his gratitude for 

 the privilege of using this material and for generous assistance, which 

 was never withheld. He also wishes to acknowledge the valuable aid 

 of Mr. Elmer Roberts, in the preparation of the manuscript. 



Most of the m.anuscript was written and most of the data were 

 analyzed at the College of Agriculture of the University of Illinois, 

 to which the author is deeply indebted for liberal use of time and 

 facilities. 



1. THE SYSTEMATIC POSITION OF THE PARENT RACES. 



This paper is based on a study of the wild Brazilian guinea-pig, 

 (Cavia rufescens Lund), the common domestic guinea-pig {Cavia por- 

 cellus Linn.), hybrids between these, and subsequent progeny obtained 

 in the next eight generations by various matings. About 1,800 animals, 

 wild or hybrid, enter in one way or another into experiments on color, 

 growth, size, and fertility. Besides these, approximately 600 guinea- 

 pigs, living under the same conditions in collateral experiments, serve 

 as a basis for necessary comparisons. 



That the hybrids are the result of a species cross rather than a 

 variety cross can hardly be doubted, since the I wild and \ wild males 

 are entirely sterile. In order to meet any doubt or criticism at the 

 outset, I may briefly give my reasons for assigning the parent stocks 

 to such diverse and distantly related species. In the summer of 1903 

 Dr. W. E. Castle received one wild male and two wild females from 

 Mr. Adolph Hempel, Campinas, Sao Paulo, Brazil. These and their 

 progeny were kept for some time at the Harvard Zoological Laboratory, 

 and were removed later to the Laboratory of Genetics, Bussey Insti- 

 tution, Harvard University. In the summer of 1911, three years after 

 the last animal of pure wild pedigree had died, we again received from 

 Mr. Hempel one wild male and one wild female. At first it was thought 



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