INBREEDING AND CROSSBREEDING 235 



ity in all respects, which in economic animals or plants is 

 very undesirable. For the characters in which the two pure 

 breeds differed undergo recombination in all possible ways 

 in the F2 offspring. Even a back-cross of an F; individual 

 with one of the pure races would produce offspring quite 

 variable and including undesirable combinations, since each 

 Fi individual would form the maximum number of different 

 kinds of gametes. Hence crossing of pure breeds of domes- 

 ticated animals may in special cases be advantageous but 

 should never be carried beyond the Fi generation unless the 

 breeder is setting out on the slow and tedious process of pro- 

 ducing and fixing a wholly new breed. In that case he must 

 be prepared to produce and sacrifice many worthless animals 

 for the sake of obtaining in the end a few of possibly superior 

 value. For such an undertaking the imagination and the 

 patience of an inventor are required. 



4. When animals or plants of widely separated species or 

 genera are crossed, one of two results follows: Either the 

 offspring are of remarkable vigor but of impaired fertility, 

 or the offspring lack both vigor and reproductive capacity. 

 In the former category comes one very important economic 

 cross, that of the horse with the ass, producing a very valu- 

 able animal, the mule. The economic importance of mules 

 is indicated by the large numbers produced in the United 

 States, South America, Europe and Africa, and by the fact 

 that the market price of a mule averages higher than the 

 price of either a horse or an ass. Nevertheless a mule is 

 absolutely incapable of reproduction. It has well developed 

 sexual glands and sexual instincts, but the sexual cells de- 

 generate before reaching full maturity. If mules were capable 

 of reproduction, they would probably be less valued than 

 they now are, for F2 and F3 individuals would doubtless then 

 be produced, and these would lack the uniformity and vigor 

 of the Fi individuals which alone exist at present. 



Crosses of cattle with the American bison produce hybrids 

 which are sterile in the male sex only, the females being fertile 

 with either parent species. By use of these fertile female 



