Before Darwin—that is, before 1st July 1858—the origin of the different species of animals and plants was called by great thinkers like Sir John Herschel, the astronomer, “the mystery of mysteries.” The word “species” was defined as “an animal or plant which in a state of nature is distinguished by certain peculiarities of form, size, colour, or other circumstance from any other animal or plant, and propagates after its kind individuals perfectly resembling the parent, its peculiarities being therefore permanent.” So wrote a great naturalist in the days before Darwin. This definition may be illustrated by two common English birds—the rook and the crow. They differ from each other in slight peculiarities of form, structure, and habits, and, moreover, rooks always 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 unknown, 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 this idea, or that their fathers considered it a scientific heresy to be condemned rather than seriously discussed.”