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- 
