COLOR AND COAT CHARACTERS. 45 



hybrids as the mule, zebroids, zebrules, and the Hke, if fertile males of 

 these classes of hybrids can be established. That fertile males can be 

 produced among these hybrids, also, is not a matter of certainty, but 

 since the female mule is reported to be occasionally fertile (Waldow 

 von Wahl 1907, Przibram 1910), it may be possible to obtain fertile 

 male mules (Detlefsen 1912). 



The detailed discussion of fertility will be given later. The color 

 inheritance of the fertile hybrid males in crosses is the same as that of 

 the females. 



11. GENERAL CONCLUSIONS AS TO COLOR AND COAT CHARACTER. 



The ancestry of the tame guinea-pig {Cavia porcellus) is a matter of 

 considerable doubt, but the prevalent opinion, based on historical and 

 morphological studies, considers the Peruvian cavy {Cavia cutleri) as 

 the immediate ancestor. The relationship between the wild Brazilian 

 cavy {Cavia rufescens) used in the foregoing experiments and the tame 

 guinea-pig is a matter of conjecture; but we may rest assured that 

 these two parent species have had no common ancestry for many, many 

 centuries at least. The relationship is distant, as shown by the many 

 differentiating characters and by the sterility involved in the cross. 



In the case of the tame-parent species, a number of unit characters 

 are well known, but in the case of the wild-parent species nothing has 

 previously been known with regard to unit characters or allelomorphic 

 pairs, for it was simply recozgnized as a wild, agouti, cavy species. 

 The tame species has many varieties, and in crossing these varieties 

 inter se we see orderly mechanical separations and recombinations of 

 allelomorphic pairs manifested in Mendelian ratios. We know of no 

 varieties in the wild species, and, since it breeds true, the natural 

 inference is that it is homozygous in most of its characters, if not all. 

 Now, in spite of the fact that these two species have been separated 

 by many centuries and thousands of miles, and by certain peculiar 

 mental and physical structures, and in spite of the many difficulties of 

 even obtaining a successful cross, finally two gametes join to form 

 the hybrid zygote. One of these gametes bears, among other things, 

 a certain number of known factors. The other gamete, coming from 

 the wild, was an unknown quantity aind one could only theorize from 

 analogy as to its constitution. To be concrete, the ova, coming from 

 the tame, carried in certain matings no agouti factor, but all the sperm 

 from the wild carried it. The hybrid zygote, therefore, carried it in 

 single dose. These contributions of the diverse parent species sepa- 

 rated in the next gametogenesis as nicely as in the case of smooth and 

 WTinkled peas, even though one sex in the first two hybrid generations 

 was completely sterile. From time to time doubts are expressed as 

 to whether Mendelian laws hold in the cases of wide crosses. One 

 purpose of these experiments has been to study a wide mammalian 



