GENERAL INTRODUCTION. 9 



It would be equally difficult to formulate any rule by which we 

 could determine how great must be the difference in color, shape, size, 

 tooth formation, and the like, between genuine "species," but for our 

 present purposes this, fortunately, is unnecessary. 



The reasons for considering the wild stock used in these experiments 

 to be specifically distinct from the guinea-pig are as follows: 



(1) The skull characters, size, and color of our wild stock undoubt- 

 edly place it in the species rufescens. I am indebted to Dr. G. M. 

 Allen for a corroboration of this classification. 



(2) Hybrids between our wild stock and the guinea-pig are sterile in 

 the male sex, regularly through two blood dilutions and in many cases 

 through more blood dilutions. 



The other parent species, the common domestic guinea-pig, Cavia 

 porcellus (also called Cavia cobaya),is too well known to require identi- 

 fication or description. The peculiarities of the stock used in these 

 experiments, if there be such, are described in the detailed discussion 

 of their inheritance. The ancestors of the guinea-pigs, in these experi- 

 ments, were obtained by purchase from dealers and fanciers, but the 

 animals which were used were of known zygotic color formulae, size 

 variabihty, and fertihty. 



2. MATERIALS AND METHODS. 

 THE WILD RACE. 



The original wild c? 1 was mated to wild 9 9 2 and 3, to increase the 

 stock. (See fig. 1.) He was Ukewise mated to his daughters, as were his 

 sons, cf24 and cTSS, and his grandson cf56. The young of 9 2 died 

 prematurely, and so do not figure in any of the later crosses; hence 

 aU the wild stock came from two original parents, cf 1 and 9 3. The 

 pure wild line eventually died out, for, even with the greatest care and 

 experience in handling domestic cavies, it was not possible to carry 

 the wild stock more than 5 years in captivity. The animals were 

 prone to fight. Only one female could be penned with one male at 

 the same time. The total number born in captivity was 46, but of 

 these only 4 females and 3 males reached sexual maturity. Our experi- 

 ence does not agree with that of Nehring (1894), who reaUzed little 

 difficulty with Cavia aperea in captivity. This fact again distinguishes 

 the two stocks and experiments. The two wild cavies received in 1911 

 have not bred up to the time of writing. 



ONE-HALF WILD HYBRIDS. 



The original wild male, cfl, and his sons, cf24 and d'SS, and his 

 grandson, cf 55, were used to obtain hybrids between the pure wild 

 stock and the tame guinea-pig. (See fig. 2.) The reciprocal cross 

 (tame male X wild female) was not obtained or even attempted, for 

 it was feared that such small females might die in pregnancy when 



