Evolution: Darwin's idea -- evolution proceeds when a new heritable trait emerges in some organism and is passed genetically to its descendants. The idea indicates that two groups of animals that share a set of these new or "derived" traits are more closely related to each other than they are to groups that display only the original traits but not the derived ones. Of course these traits can be lost subsequently, and similar traits can evolve more than once, but tracking these traits is the best way to work out the phylogenetic relationships among organisms. We may track more than one trait, such as both feathers and skeletons. Results of these analytical tracks is a "cladogram," a treelike diagram that describe the order in which new traits and new creature evolves. Each branching point reflects the emergence of a ancestor that founded a group having derived characters that were not present in the group from which it evolved. The ancestor and its descendants constitute a branch in the tree; root word "clade" - branch. This style of reasoning is cladistics. The groups are hierarchial in a historical sense, a branch includes all animals that shared a recent common ancestor. "What is a bird" is a historical question.Look at fossil birds, and other fossils that had bird-like features. These are of interest in determining the age of birds, the ancestors of birds, the series of changes that birds underwent in evolution, whether modern birds are related to the radiation of fossil birds in the Mesozoic, and what makes a bird a bird. We shall consider whether feathers make the bird, or whether another definition of birds is more appropriate