PREFATORY NOTE BY W. E. CASTLE. 



In July 1903 I received from Mr. Adolph Hempel, of Campinas, 

 Brazil, three wild cavies, a male and two females, of a species supposed 

 at the time to be Cairia aperea, but now referred to Cavia rufescens. 

 The male and one of the females bred in captivity and produced a 

 considerable number of descendants, certain of which (together with 

 the original male) were employed in crosses with ordinary guinea-pigs. 

 The hybrids thus obtained proved completely sterile in the male sex, 

 but the females were entirely fertile. Further propagation of the 

 hybrid race was thus restricted to crossing the female hybrids with 

 males of one of the parent species. 



In December 1909 I turned over to my assistant, J. A. Detlefsen, 

 for further study, the stock of hybrid animals, together with the pedi- 

 gree records and notes of such observations as I had been able to make 

 upon the hybrid race. The present paper will indicate how successful 

 he has been in propagating the hybrid race and what conclusions may 

 be drawn concerning the inheritance of various characters in these 

 hybrids. 



The long series of experiments upon which a partial report is here 

 made was rendered possible by a grant from the Carnegie Institution 

 of Washington and by the provision of special facilities on the part of 

 Harvard University. Grateful acknowledgment is made of my obliga- 

 tion to both institutions. 



