78 A NEW THEORY OF EVOLUTION. 



"without any cause" or "apparently from 

 mere inability to live." The same results 

 follow the cross-breeding of dogs. The 

 lurcher is a cross between the greyhound 

 and the collie, and combines in high degree 

 the swiftness and keenness of vision of the 

 greyhound and the intelligence and scent- 

 ing faculty of the collie ; but no breeder 

 thinks of breeding lurchers from lurchers, 

 because the degeneracy of their mongrel 

 progeny is well known. 



It is not easy to see how varieties of the 

 same species can exist in the same locality. 

 According to the common standard of fertility, 

 all are fertile together, and if so, all would 

 by heredity become blended together in ex- 

 pression, and variety would cease. In our 

 opinion, varieties in the same locality are 

 distinct races, kept pure by that aversion 

 between races that we know exists without 

 any satisfactory explanation unless to pre- 

 serve purity of race between different races 

 of mankind, and very notably between some 

 species of lower animals. 



But it may be asked how the distinct 

 breeds of pure cattle, now registered in their 

 respective herd-books, have appeared within 

 the last century? Are they not examples 

 of new varieties ? 



