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CHAPTER IV. 



ANALOGY, HOMOLOGY, HOMOMORPHISM, MIMICRY, AND 

 CORRELATION OF GROWTH. 



I. ANALOGY. The term "analogue" was defined by 

 Owen to be "a part or organ in one animal which has the 

 same functions as another part or organ in a different ani- 

 mal." In other words, those parts or organs are analogous 

 which resemble one another physiologically and discharge 

 the same functions, wholly irrespective of what their funda- 

 mental structure may be. In most cases the organs which 

 would ordinarily be called " analogous" are such as differ 

 from one another in structure, at the same time that they 

 discharge the same duties. Thus the wings of a bird and 

 the wings of an insect are analogous organs, since they are 

 both organs of flight, and serve to sustain their possessor in 

 the air. They are, however, in no way similar to one an- 

 other except when regarded from this physiological point of 

 view; and they differ altogether from a morphological aspect, 

 being in no way formed on the same fundamental plan. It 

 often happens, however, that " analogous " organs have the 

 deeper relation to one another of being constructed upon 

 the same morphological plan, in which case, in addition to 

 their analogy, we have to consider the relationship which is 

 known by the general name of " homology." 



II. HOMOLOGY. According to Owen, a "homologue" is 

 " the same organ in different animals under every variety of 



