CLASSES OF ANIMALS. 227 



lar system, to their digestive apparatus, to their circula- 

 tion, 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 Mol- 

 lusks, 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 illustrating com- 

 parative anatomy. 



Finally, it ought to be remembered, in connexion with 

 the study of classes as well as that of other groups, that 

 the amount of difference existing between any two divi- 

 sions 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 unequal weight of different groups, so evident every- 

 where in the animal kingdom, ought to make us more 

 cautious in estimating their natural limits, and pre- 

 vent us from assigning an undue value to the differences 

 observed between living beings, either by overrating appa- 

 rently great discrepancies, or by underrating seemingly 

 trifling variations. The right path, however, can only be 

 ascertained by extensive investigations 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 ob- 

 served between species is constantly urged, even without 

 a preliminary investigation, as an argument for distin- 

 guishing them. These differences, moreover, are not only 

 quantitative, they are to a still greater extent also quali- 

 tative. In the same manner do genera differ more or less 



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