HOMOLOGY AND ANALOGY. 23 



development, repeats the most important of those changes of 

 form which its ancestors have passed through during the long 

 and gradual course of their palaeontological development, in 

 accordance with the laws of transmission and adaptation." 



8. HOMOLOGY, ANALOGY, AND HOMOMORPHISM. 



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 j 

 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 appen- 

 dages being constructed upon the same type, and being there- 

 fore 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 walk- 

 ing, 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 ex- 

 cellent instance of this is seen in the common Millipede (iulus). 



Homomorphism. Many examples occur, both among animals 

 and among plants, in which families widely removed from one 

 another as to their fundamental structure, nevertheless pre- 

 sent a singular, and sometimes extremely close, resemblance 

 in their external characters. Thus the composite Hydroid 



