74 EVOLUTION AND NATURAL THEOLOGY. 



A character which has once appeared in a 

 race, and has, so to speak, been " bred out," by 

 repeated crosses with individuals not possessing 

 it, has always a certain tendency to reappear 

 at some future period in its descendants, even 

 after an indefinite number of generations. 

 This is called "reversion," a term which we 

 shall frequently have occasion to employ. 



Thus it is now admitted by all acquainted 

 with the subject, that our domestic pigeons, 

 which vary to such an extent as to exhibit 

 characters unique in the entire Class of birds, 

 have all descended from the wild rock-pigeon. 

 Specimens occasionally appear in all the domes- 

 tic breeds which reproduce the colouring of the 

 wild species ; and this tendency is much in- 

 creased by crossing individuals of different races.* 



The subject of hybridism, alluded to in the 

 last chapter, here demands further consideration. 

 When species are crossed, they are usually 

 sterile ; but sometimes produce infertile off- 

 spring. Thus the mule, though generally sterile, 

 has occasionally been known to breed, showing 



* Darwin, " Origin of Species," pp. 177 180. "Animals and Plants 

 under Domestication," vol. i. 



