1 8 MANUAL OF ZOOLOGY. 



in structure and in function, the case becomes rather one of 

 what is called " vegetative " or " irrelative repetition." An ex- 

 cellent instance of tJiis 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 character's. 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 EuphorbiiE 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 certaii! 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 ex- 

 amples of " mimicry," and such animals are said to be " mim- 

 etic." Excellent examples of this may be found amongst 

 certain Butterflies, or in the close resemblance of the clear- 

 winged Moths to Bees and Hornets. In all these cases it 

 appears that the mimetic species is protected from some enemy 

 by its outward similarity to the form which it mimics. Finally, 

 there are numerous cases in which animals mimic certain 

 natural objects, and thus greatly diminish their chances of 

 being detected by their natural foes. Excellent instances of 

 this are afforded by the insects known as Walking-leaves {Phyl- 

 lium) and Walking-sticks {Phasmida), which respectively 

 present the most singular resemblance to leaves and dried 

 twigs. 



8. Cor.RELATION OF GROWTH. 



This term is employed by zoologists to exprecs the empi- 



