272 ESSAY ON CLASSIFICATION. 



features of their structure. We now proceed to examine 

 the more remote and less definite relations, which are 

 called analogies. 



It has already been stated in what way homologies dif- 

 fer from analogies. 1 Homology is that kind of relation- 

 ship which is founded upon identity of structure in differ- 

 ent animals belonging to natural divisions of the same 

 kind; while analogy is a resemblance arising from the 

 combination of features characteristic of one natural 

 group with those of another group. 2 We have, indeed, 

 seen that all the animals belonging to the same branch 

 are homologous, as far as the plan of their structure is 

 concerned; that all the members of the same class are 

 homologous, as far as the mode of execution of that struc- 

 ture is concerned ; that all the members of the same order 

 are homologous in the complication of their structure; 

 that all the representatives of the same family are homo- 

 logous in form; that the different genera of one and the 

 same family exhibit homologous peculiarities in the de- 

 tails of their structure ; and that even within the narrow 

 limits of species we may still trace homologous features, 

 among the genera which have numerous representatives, 

 even when such resemblances do not extend to the species 

 of closely allied genera. It is plain from this that the 

 categories of homology are as numerous and diversified 

 as the essential kinds of differences which we may trace 

 in the structure of animals; or, in other words, we have 

 branch homologies, class homologies, ordinal homologies, 

 family homologies, generic homologies, and specific homo- 



1 See p. 26. based upon similarity of function, 



" Houiulogy has also been defined without reference to structure. The 



as (he relationship arising from definition given above is more pre- 



identity of structure without refer- cise, as it embraces all the different 



euce to function, while analogy is categories of analogy and homology. 



