HOMOLOGY, ANALOGY, AND HOMOMORPHISM. 1 7 



each sub-kingdom in their adult condition are truly referable 

 to the degree to which development proceeds, the place o( 

 each individual in his own sub-kingdom being regulated by the 

 stage at which development is arrested. Thus, many cases 

 are known in which the younger stages of a given animal 

 represent the permanent adult condition of an animal some- 

 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 Aclult Pteropod. 



7. Homology, Analogy, and Homomorphism. 



When organs in different animals agree with one another ui 

 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 

 diiferent regions of the body to enable them to fulfil special 

 functions, some being adaptfed 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 



