DARWIN'S THEORY 15 



produce rooks, and crows always produce crows, and 

 they do not interbreed. Therefore it was held that all 

 the rooks in the world had descended from a single pair 

 of rooks, and all the crows in like manner from a single 

 pair of crows, while it was considered impossible that 

 crows could have descended from rooks, or rooks from 

 crows. The " origin " of the first pair of each kind was 

 a mystery, and by many persons was held to have been 

 due to a miraculous and sudden act of " creation." But 

 besides our crow and rook, there are about thirty other 

 birds in various parts of the world so much like our crow 

 and rook that they are commonly called crows, and are 

 all regarded as " species " of the genus crow (or Corvus). 

 It was held before Darwin that all the individuals of each 

 of these " species " were descended from an ancestral pair 

 of crows of that species. There would have been thirty 

 different original kinds, the " origin " of which was un- 

 known, and by naturalists was regarded as a mystery. 

 Now, on the contrary, it is held that all the thirty living 

 species are descended from one, not from thirty, ancestral 

 species, and have been gradually modified to their present 

 character in different parts of the world ; and, further, 

 that this ancestral species was itself derived by slow 

 process of change and natural birth from preceding crow- 

 like birds no longer existing. 



As Mr. Alfred Russel Wallace has said in his most 

 readable and delightful book, Darwinism where he 

 gives all the credit and glory to his great fellow-worker : 

 " Darwin wrote for a generation which had not accepted 

 evolution a generation which poured contempt on those 

 who upheld the derivation of species from species by any 

 natural law of descent. He did his work so well that 

 'descent with modification' is now universally accepted 

 as the order of nature in the organic world, and the rising 

 generation of naturalists can hardly realise the novelty of 



