278 ESSAY ON CLASSIFICATION. 



appendages of the mouth of Limulus are only analogous 

 to the legs of the Decapods, as far as their form is con- 

 cerned, these organs are yet homologous as parts of the 

 body of an Articulate. This and similar cases may shew 

 how wide a field of investigation lies before us in the 

 study and discrimination of homological and analogical 

 forms. 



Generic Analogies. As the generic characters are 

 based upon peculiarities of structure limited to some part 

 or other of an animal, we may expect to find the generic 

 analogies reduced to a resemblance of certain parts of the 

 body and not extending to its general appearance. For 

 while genera, as members of a family, must exhibit the 

 same form, combined with the structural complication of 

 their order, it is obvious that, if there is any generic ana- 

 logy between animals of different families, their whole 

 form may be widely different and the complication of 

 their structure exhibit entirely different combinations, or 

 be based upon different modes of execution, if they 

 belong to different classes, and even be constructed upon 

 different plans of structure, if they belong to different 

 branches; and yet some of their parts should be similar 

 in some way or other, in order to present a generic 

 analogy. 



Now such generic analogies are rather frequent, and 

 may be traced between animals of widely different fami- 

 lies, belonging to different orders, nay even to different 

 classes and to different branches ; for instance, there is 

 a marked generic analogy between the dentition of the 

 Insectivora, of the class of Mammalia, and that of the 

 Characini of the class of Fishes, so also between some 

 genera of the family of Sparoids and those of the Chro- 

 mids, between some genera of the family of Insectivora 



