1 6 MANUAL OF ZOOLOGY. 



what lower in the scale. To give a single example, the young 

 Gasteropod (amongst the Mollusca) transiently presents all the 

 essential characters which permanently distinguish the adult 

 Pteropod. The development of the Gasteropod, however, pro- 

 ceeds beyond this point, and the adult is much more highly 

 specialised than is the adult Pteropod. 



7. 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 

 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 (lulus). 



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- 



