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. On the one hand, creationists deny this.
Natural selection is the mechanism of change.
All organisms produce more offspring than can possibly be supported by the limited resources. Adult populations remain relatively constant. As a result, all organisms must be engaged in a constant competition to acquire these limited resourses for themselves in order to survive. Darwin and Wallace realized that this process operating within species would result in organisms that appeared designed for their environment. However, although Darwin recognized the need for some sort of granular unit of heredity, he was unaware of the work of Mendel.