CiiAP. II. CLASSES OF ANIMALS. 149 



great types of the animal kingdom. While, however, homologies show the close 

 similarity of apparently different 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 Avith the trachere 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 Verte- 

 brates, as essentially different among themselves, though homologous within the limits 

 of each tyjie; 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 reproductive organs, etc. It would not 

 be difficult to show now that the alimentary canal with its glandular appendages, 

 in Vertebrata, is formed in an entirely different way from that of Articulates or 

 Mollusks, and that it cannot be considered as homologous in all these types. And 

 if this be true, we must expect soon an entire reform of our methods of ilkuitrating 

 comparative anatomy. 



Finally, it ought to be remembered, in connection with the study of classes as 

 Avell as that of other groups, that the amount of difference existing between any 

 two divisions is nowhere the same. Some features in nature seem to be insisted 

 upon with more tenacity than others, to be repeated more frequently and more 

 widely, and to be impressed upon a larger number of representatives. This 

 nnocpial weight of different groups, so evident everywhere in the animal kingdom, 

 ought to make lis more cautious in estimating their natural limits, and prevent us 

 from assiffnincj an undue value to the differences observed between liA-infr beinss, 

 never overrating apparently great discrepancies, nor imderrating seemingly trifling 

 variations. The right path, however, can only be ascertained by extensive inves- 

 tigations, made with special reference to this point. 



Everybody must know that the males and females of some species differ much 

 more one from the other than many species do, and yet the amount of difference 

 observed between species is constantly urged, even without a preliminary investi- 

 gation, as an argument for distinguishing them. These differences, moreover, are 

 not only quantitative, they are to a still greater extent also qualitative. In the 



