I 8 MANUAL OF ZOOLOGY. 



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- 

 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 resemblance of different animals 

 might be adduced, and in many cases these " representative 

 forms " appear to be able to fill each other's places in the 

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

 that " homomorphous " forms are generally found in different 

 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 

 discharge the functions elsewhere performed by several orders. 

 Many homomorphous forms, however, live peacefully side by 

 side, and it is difficult to say whether in this case the resem- 

 blance between them is for the advantage or for the disadvan- 

 tage of either. In other cases we find certain animals putting 

 on the external characters of certain other animals, to which 

 they may be closely related, or from which they may be widely 

 separated in zoological position. Such cases are said to be 



