GENERAL INTRODUCTION. 11 



as this from the original stock. At present most of our pens contain 

 ^ wild hybrids. 



The numbers of hybrids obtained up to October 1911 were as follows: 

 I wild, 39; | wild, 4; i wUd, 83; | wild, 217; rV wild, 312; ^V wild, 

 344; -sV wUd, 122; ^4^ wild, 37; ^^ wild, 2; total, 1,160. Since that 

 time 600 more hybrids have been born. 



Unfortunately, for comparisons, mammalian species crosses are not 

 common. When they have been made the number of offspring has 

 been small, thus affording small basis for generalization. The most 

 reliable data are drawn from species crosses among the ungulates, but 

 ungulates are not adapted to laboratory experiments in large numbers. 

 Species crosses are unknown among the Monotremata, Edentata, Insec- 

 tivora, Chiroptera, Sirenia, Proboscidea, and Hyracoidea (Przibram 

 1910). The species crosses among ungulates, like horse and ass, or 

 cow and bison, involve the question of sterility and fertiUty. The 

 similar sterility in the cross of the wild and tame guinea-pig affords 

 excellent material for comparison with these larger economic forms. 



FERTILE MALES IN MATINGS. 



The most interesting part of the whole problem is the origin of fertile 

 hybrid males and crosses of such males with females of the different 

 blood-dilutions and with guinea-pig females. Seven fertile males 

 appeared among our | wild hybrids. The number increased rapidly 

 in the yt wild, -jV wild, and later generations. The importance of 

 these males is apparent; for it gives opportunity to study sterility and 

 fertility, and to test whether any segregation of characters in the direc- 

 tion of Cavia rufescens is possible. Previously, any segregation possible 

 was in the direction of the guinea-pig, Cavia porcellus. The fact is 

 realized that a large number of characters is involved, and it will there- 

 fore require the observation of many individuals before we can reason- 

 ably expect to observe complete segregation of either the guinea-pig 

 or the rufescens characters as a group. Fortunately many of the 

 characters are so unmistakable and definite as to allow of no doubt or 

 uncertainty in their case. The detailed result of the matings of the 

 fertile male hybrids is given in Part III. The young from such mat- 

 ings have not reached maturity and consequently their bone measure- 

 ments and growth curves can not be given at this time. 



ACCUMULATION OF DATA. 



It has been stated that a number of differential characters mark the 

 wild guinea-pig in distinction from the tame. Records of the expres- 

 sion of these characters and new characters which appeared have been 

 made. 



Color. — Cavia rufescens is not known to occur in any color, except 

 agouti of a rather distinct and specific type. Color records of each 

 hybrid were made at birth. 



