When organs in different animals agree with one another in fundamental structure, they are said to be "homologous;" when they perform the same functions they are said to be "analogous." Thus the wing of a bird and the arm of a man are constructed upon the same fundamental plan, and they are therefore homologous organs. They are not analogous, however, since they do not perform the same function, the one being adapted for aerial locomotion, the other being an organ of prehension. On the other hand, the wings of a bird and the wings of an insect both serve for flight, and they are therefore analogous, since they perform the same function. They are not homologous, however, as they are constructed upon wholly dissimilar plans. There are numerous cases, however, in which organs correspond with one another both structurally and functionally, in which case they are both homologous and analogous.

A form of homology is often seen in a single animal in which there exists a succession of parts which are fundamentally identical in structure, but are variously modified to fulfil dif- ferent functions. Thus a Crustacean - such as the lobster - may be looked upon as being composed of a succession of rings, each of which bears a pair of appendages, these appendages being constructed upon the same type, and being therefore homologous. They are, however, variously modified in different regions of the body to enable them to fulfil special functions, some being adapted for swimming, others for walking, others for prehension, others for mastication, and so on. This succession of fundamentally similar parts in the same animal constitutes what is known as serial homology. When, however, the successive parts are similar to one another, both in structure and in function, the case becomes rather one of what is called "vegetative" or "irrelative repetition." An excellent instance of this is seen in the common Millipede (Iulus).