LEADING GROUPS OF EXISTING SYSTEMS 153 



compare more closely the structure of the different classes of the 

 animal kingdom, points of resemblance have been noticed between 

 them which had entirely escaped the attention of earlier investigators, 

 structural modifications have been identified, which at first seemed 

 to exhibit no similarity, so much so, that step by step these com- 

 parisons have been extended over the whole animal kingdom, and it 

 has been asserted that whatever may be the apparent differences in 

 the organization of animals, they should be considered as constructed 

 of parts essentially identical. This assumed identity of structure has 

 been called homology. ^^ But the progress of science is gradually re- 

 stricting 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 sho^v the close similarity of apparently differ- 

 ent structures and the perfect 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 identical 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 homologous 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 respiratory organs in Mollusks, 

 in Articulates, and in Vertebrates 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 muscular system, to 

 their digestive apparatus, to their circulation, and to their reproduc- 

 tive organs, etc. It would not be difficult to show now that the ali- 

 mentary canal with its glandular appendages in Vertebrata is formed 



" See Chap. I, Sect. v. 



