226 ESSAY ON CLASSIFICATION. 



animals, they should be considered as constructed of parts 

 essentially identical. This assumed identity of structure 

 has been called homology. 1 But the progress of science 

 is gradually restricting these comparisons within narrower 

 limits, and it appears now that the structure of animals is 

 homologous only as far as they belong to the same branch, 

 so much so, that the study of homologies is likely to 

 afford one of the most trustworthy means of testing the 

 natural limits of any of the great types of the animal 

 kingdom. While, however, homologies show the close 

 similarity of apparently different structures and the per- 

 fect identity of their plan within the same branches of the 

 animal kingdom, yet they daily exhibit more and more 

 striking differences, both in plan and structure, between 

 the branches themselves, leading to the suspicion that 

 systems of organs which are generally considered as iden- 

 tical in different types, will, in the end, prove essentially 

 different, as, for instance, the so-called gills in Fishes, 

 Crustacea, and Mollusks. 



It requires no great penetration to see already that the 

 gills of Crustacea are homologous with the tracheae of 

 Insects and the so-called lungs of certain spiders, in the 

 same manner as the gills of aquatic Mollusks are homo- 

 gous with the so-called lungs of our air-breathing snails 

 and slugs. Now, until it can be shown that all these 

 different respiratory organs are truly homologous, I hold 

 it to be more natural to consider the system of respira- 

 tory organs in Mollusks, in Articulates, and in Verte- 

 brates as essentially different among themselves, though 

 homologous within the limits of each type ; and this 

 remark I would extend to all their systems of organs, to 

 their solid frame, to their nervous system, to their muscu- 



1 See Chap. I, Sect. 5. 



