Systematists, comparative biologists, taxonomists and evolutionary biologists all concern themselves with the evolutionary relationships between animals and plants. Homology is the principle underlying these disciplines. When looking at groups of organisms, similarities between them (homologies) suggest a close common ancestor. For example, human and porcine insulin is very similar in structure (porcine insulin was once used to treat human diabetics). However, there are usually characteristics which would oppose such theories. For example: totally different foetal development, and the...