EOMOLOGY, ANALOGY, AND HOMOMORPHISM. 15 



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 

 excellent instance of this is seen in the common Millipede 

 (lulus). 



HomomorpMsm. 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 

 Polypes and the Polyzoa are singularly like one another ; 

 so much so, that they have often been classed together, 

 whereas, in reality, they belong to different sub-kingdoms. 

 Many other cases of this * mimetic ' resemblance of different 

 animals might be adduced, and in many cases these ' repre- 

 sentative forms ' appear to be able to fill each others' places 

 in the general economy of nature. This is so far true, at any 

 rate, that ' homomorphous ' forms are generally found in dif- 

 ferent parts of the earth's surface. Thus, the place of the 

 Cacti of South America is taken by the Euphorbias of Africa ; 

 or, to take a zoological illustration, many of the different orders 

 of Mammalia are represented in the single order Marsupialia 

 in Australia, in which country this order has almost alone to 



