THE CATEGORIES OF ANALOGY. 273 



logics. Examples of tlie more comprehensive kinds of 

 these homologies will occur to every practical geologist. 

 As to specific homologies, they are particularly traceable 

 in those structural features which determine the propor- 

 tions among the parts; as for instance, when all the 

 species of one genus are either long-necked, short-tailed, 

 long-legged, etc., while those of closely allied genera may 

 present reverse proportions, etc. 



Let us now see what are the categories of analogy, and 

 how far it is possible, under all circumstances, to distin- 

 guish homological and analogical features. If analogy is a 

 resemblance arising from a combination of features charac- 

 teristic of one group, with those characteristic of another 

 group (such as class characters of one class with those of 

 another class, or those of families of another class), then the 

 investigation will only require the recognition of the dif- 

 ferent categories of structure already considered (such as 

 branches, classes, orders, etc.), and a correct appreciation 

 of the mode of their combination with those of another 

 group. It will, for instance, be sufficient to ascertain in 

 what manner the features, resulting from a certain mode 

 of execution of the homologies of one type, are combined 

 with structures of another type; or, in other words, to 

 recognize any feature wherever it appears, and not merely 

 within the limits within which structures are strictly 

 homologous. The study of analogies is therefore limited 

 to the investigation of more or less distinct features that 

 are naturally characteristic of one kind of group, in their 

 combination with features of groups of another kind. For 

 instance, the similarity between an insect wing and the 

 w r ing of a bird is based upon analogy. The entire differ- 

 ence of structure between the organs of flight in these 

 two classes of animals forbids our considering the resem- 



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