Creationists may acknowledge that very similar species evolve from a common ancestor, but they deny that new "kinds" of organisms can evolve. One creationist, Duane Gish, has defined a "kind" as "including all those variants which have been derived from a single stock". This stock, he holds, gives rise to variants, but not to different "kinds". He notes that, "we cannot always be sure... what constitutes a separate kind," but cites fishes, amphibians, reptiles, birds, mammals, and within the mammals dogs, cats, monkeys, chimpanzees, gorillas, and humans as "different basic kinds". What is the validity of the concept of a "kind"? Evaluate this creationist claim in light of what you know about the evidence of evolution.