It should be clear that the typical creationist argument that a dog has never been observed to give birth to a cat, or a monkey to give birth to a human is a nonsense straw man. Evolutionary theory does not now, and never has claimed such drivel. Darwin did propose that intermediates between many living organisms were erased by extinction, and the evidence seems to indicate that he was correct. Creationists point to these gaps. Individual organisms do not evolve and offspring are not different species from their parents.
Evolution is a process that occurs in the heritable characteristics of a population over time.
Even though Darwin knew nothing of genetics, this concept has become the very definition of evolution at the most fundamental level. Creationists snidely wonder if they should have a net handy in case their next child is born with wings and feel that this is a telling argument against evolution. But if such an event occurred, it would be an overwhelming blow to current theories of evolution. Any new structure is always a slight modification of a previously existing structure. He was the first to argue that species had diverged from common ancestors and thus comprise a great tree of life.