PRINCIPLES OF LIVESTOCK BREEDING. 11 



of crosses between the sheep and hog may be stated confidently to 

 have no foundation. It is even doubtful whether hybrids can be 

 produced from two such closely related animals as the sheep and the 

 goat, or the dog and the fox. A few possibly authentic instances have 

 been reported, but at best a successful cross appears to be exceedingly 

 unusual in both these cases. The European breeds of cattle cross 

 freely with the Indian humped cattle, although the latter are con- 

 sidered to be of a different species. The cross with the American 

 bison has often been made, the progeny being called cataloes. Some 

 of the females are fertile, but the few males born alive have been 

 sterile. Fertile males have been obtained by backcrossing with 

 the parental species. 



The most important species cross among mammals is, of course, 

 that of the horse and ass. Both sexes of the mule, produced by a 

 jack and a mare, and of the hinny, produced by a stallion and a jennet, 

 are probably always sterile. There are occasional reports of fertile 

 mare mules, but none of these seem to have been established beyond 

 doubt. Both the horse and ass will cross with the various species of 

 zebra, producing hybrids which so far as known are always sterile. 

 A number of sterile hybrids have been produced in crosses between 

 the domestic fowl, guinea fowl, peacock, pheasant, etc. 



THE REPRODUCTIVE CELLS IN RELATION TO HEREDITY. 

 GENERAL CONSIDERATIONS. 



As already noted, every individual begins his career in the union of 

 two reproductive cells. All that is inherited from his ancestors is 

 somehow passed on by these microscopic bits of living jelly. Any 

 attempt to understand heredity should thus begin with a considera- 

 tion of these cells and their mode of production by the parents. 



At one time it was supposed that the reproductive cells were pro- 

 duced in some way by contributions from all parts of the body, 

 building up, as it were, a miniature organism, ready to develop into 

 an adult under the proper conditions. The egg and sperm cells were 

 thus supposed to transmit the characteristics of the parents as they 

 were at the time of their production. It was taken for granted that 

 the powers of hereditary transmission of an individual could be 

 changed by training, care, or even accident in such way that his subse- 

 quent offspring would show a special tendency to develop the new 

 characteristics. 



This view was first seriously questioned when it was found that 

 the reproductive cells, like all other cells in the body, are produced 

 only by the division of previous cells. Certain cells remain un- 

 specialized from the beginning of development and after repeated 

 divisions produce the reproductive cells and these only. The remain- 



