Interbreeding of Varieties of Beasts and Birds, 77 



are maintained by interbreeding. For instance, sparrows and 

 other finches with white wing quills may be seen breeding with 

 normally attired individuals ; and no doubt the silver-grey, 

 like the cross fox, freely cohabits with the red fox of North 

 America. 



It is therefore probable that with extremes only there is any 

 great repugnance to intercourse. Nevertheless we observe 

 a strong disposition in certain species, — nay, even genera and 

 families of animals — to produce albino and black varieties. 

 Of the latter, both foxes and squirrels furnish good examples, 

 to which further reference will be made in the next chapter. 



