CHAPTER IV. 



GENERAL LAWS OF ANIMAL STRUCTURE, OR GENERAL 

 LAWS OF MORPHOLOGY, OR PHILOSOPHICAL ANATOMY. 



SECTION I. 

 Introductory. 



This is a subject of fascinating interest in many 

 ways, but especially on account of its bearing on the 

 theory of the origin of organic forms by evolution. It 

 is for this reason mainly that we shall treat it some- 

 what fully. 



I find it necessary, however, to make some prelimi- 

 nary definitions of terms. 



Analogy versus Homology. — Parts or organs in 

 different animals are said to be analogous when they 

 have a similar form, and especially when they perform 

 a similar function. Contrarily, parts or organs of dif- 

 ferent animals are said to be homologous when, however 

 different their general apearance and however different 

 their functions, they can be shown to have a common 

 origin — to be, in fact, the same part, only modified in 

 order to perform different functions. The difference 

 between these can be best shown by examples ; and the 

 ideas involved in these terms lie at the basis of all I 

 shall say. We will give examples from plants, as well 

 as animals. 



Examples: Animals. — i. The wing of a bird and the 

 wing of a butterfly are analogous organs. They have a 

 similar flat form, adapting to a similar function — viz., 



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