154 ESSAY ON CLASSIFICATION 



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 illustrating comparative anatomy. 



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

 of classes as well as that of other groups, that the amount of differ- 

 ence 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 unequal 

 weight of different groups, so evident everywhere in the animal king- 

 dom, ought to make us more cautious in estimating their natural 

 limits and prevent us from assigning an undue value to the differ- 

 ences observed between living beings, never overrating apparently 

 great discrepancies nor underrating seemingly trifling variations. The 

 right path, however, can only be ascertained by extensive investiga- 

 tions 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 investigation, as an argument for 

 distinguishing them. These differences, moreover, are not only quan- 

 titative, they are to a still greater extent also qualitative. In the 

 same manner do genera differ more or less one from the other, even 

 in the same family; and such inequality, and not an equable appor- 

 tionment, is the norm throughout nature. In classes it is not only 

 exhibited in the variety of their forms, but also to an extraordinary 

 extent in their numbers, as, for instance, in the class of Insects com- 

 pared to that of Worms or Crustacea. The primary divisions of the 

 animal kingdom differ in the same manner one from the other. 

 Articulata are by far the most numerous branch of the whole animal 

 kingdom; their number exceeding greatly that of all other animals 

 put together. Such facts are in themselves sufficient to show how 

 artificial classifications must be which admit only the same number 

 and the same kind of divisions for all the types of the animal king- 

 dom. 



