CROSS-BREEDING 249 



234. Crossing species. Many species may be suc- 

 cessfully crossed. Some of these crosses are of great 

 economic value, as the cross between the mare and the jack. 

 The number of successful crosses between animal species 

 is not large. Such unions are difficult to make and 

 generally sterile. When such crosses are possible and 

 the union is fertile, the offspring is generally partially 

 or wholly sterile. Some of the successful crosses which 

 have been reported are the sheep and goat, horse and ass, 

 horse and zebra, cattle and yak, cattle and bison, brahmin 

 and domestic cow, game cock and guinea fowl, domestic 

 fowl and pheasant, dog and wolf, and dog and fox. 



235. Crossing bison and cattle. A most interesting 

 experiment in cross-breeding between the bison and 

 domestic cattle is reported by Mossom M. Boyd. 1 The 

 hybrid offspring from Hereford dams and bison sire were 

 very uniform, all having white faces, were larger than 

 the bison and much smoother, broader and deeper than 

 the sire. Great difficulty was experienced in making 

 the first cross from the excessive secretion of the amniotic 

 fluid. This difficulty caused many deaths. The per- 

 centage of males from the first cross was very small. 

 Among forty-five hybrids, only six were males. Of these 

 three died at birth, one died in less than twenty-four hours 

 after birth, one proved barren, and the last male was 

 killed before determining his fertility. Charles Good- 

 night of Texas reports 2 that " no male calves have ever 

 been born; cows conceiving them either suffer abortion 

 or die, hence only get heifer calves and only a small per 

 cent of them." The hybrids produced their first calves 

 at an average age of five years (Plate XVII). 



1 Boyd, Journal of Heredity, 1914, p. 189. 



2 Goodnight, Journal of Heredity, 1914, p. 199. 



