SYSTEMS OF BREEDING 609 



Disadvantages of crossing (hybridizing). The difficulty of 

 securing a blend out of a violent cross, or indeed anything 

 that will breed pure, and the great mass of long-continued and 

 disappointing reversions experienced, have turned the attention 

 largely away from this system of breeding, to one which, if less 

 spectacular, is eminently safer, and, so far as we now know, 

 fully as fruitful of results. 



It is the opinion of the writer, however, that as we learn by 

 experience it will be found that certain races of plants will lend 

 themselves well to this means of producing new varieties, and 

 that the old-time enthusiasm for hybridization will return in 

 these exceptional cases. 



Crossing is a powerful means of inducing variability, indeed, 

 it is the most powerful method known to breeders. It is alto- 

 gether too fruitful of variants to be manageable in animal breed- 

 ing, and only sheer necessity, after all other methods have failed, 

 would warrant its trial among these slow-breeding races. 



If animals are to be hybridized it can probably best be 

 accomplished by combining, not two races simply, but three or 

 more, leaving the one nearest that which is wanted untouched 

 until a fairly favorable cross between two others has been secured. 

 Then the pure form, if bred with the cross, might be influenced 

 thereby, but would of course remain prepotent. Such a plan of 

 action aims rather at the modification of a breed than at the 

 creation of a new one. 



Hybrids often sterile. All degrees of productivity are found 

 in hybrids, from extreme fertility to absolute sterility. Some 

 crosses are more fertile than either parent. Such a cross would 

 be made readily in nature. Others are absolutely or nearly sterile. 

 It is safe to assume that about all the possible fertile hybrids 

 were long ago produced in nature, and either went down under 

 natural selection, or became good species before they came into 

 our hands. However, modified strains may yet be hybridized, 

 and sterile hybrids may often be propagated asexually. 



The classic hybrid is the mule or hinny, the cross between the 

 horse and the ass, and is nearly always sterile. The lion and the 

 tiger mate freely, in captivity at least, but the mating is in most 

 cases fruitless. Even here, however, hybrids have been born. 



