Charles Darwin's On the Origin of Species, in which he writes of his theories of evolution by natural selection, is one of the most important works of scientific study ever published." I will here give a brief sketch of the progress of opinion on the Origin of Species. Until recently the great majority of naturalists believed that species were immutable productions, and had been separately created. This view has been ably maintained by many authors. Some few naturalists, on the other hand, have believed that species undergo modification, and that the existing forms of life are the descendants by true generation of pre existing forms. "