All changes in animal form and function during evolution are due to changes in their DNA. Such changes determine which proteins are made, and where and when, during embryonic development. These proteins thus control the behaviour of the cells of the embryo. In evolution, changes in organs usually involve modification of the development of existing structures--tinkering with what is already there. Good examples are the evolution of the jaws from the pharyngeal arches of jawless ancestors, and the incus and stapes of the middle ear from bones originally at the joint between upper and lower jaws. However, it is possible that new structures could develop, as has been suggested for the digits of the vertebrate limb, but the developmental mechanisms would still be similar. It is striking how conserved developmental mechanisms are in pattern formation, both with respect to the genes involved and the intercellular signals. For example, many systems use the same positional information but interpret it differently. One of the ways the developmental programmes have been changed is by gene duplication, which allows one of the two genes to diverge and take on new functions--Hox genes are an example. Another mechanism for change involves the relative growth rates of parts of a structure.