Gradation of the Vertebrata. 139 



liiglier than an annelid, and on tlie other hand that the insectea 

 outrank the bryozoa. Yet the first is classified with the last 

 and the second with the third, the rank and the classification 

 resting on entirely different grounds. While it must be admit- 

 ted that a classification based upon a single portion of the 

 j^hysical system would be of little value, it does not follow 

 that this is true of gradation. It is true, a classification may 

 be made on the basis of rank, but it is manifest such a classifi- 

 cation would be most arbitrary and unnatural. Array all the 

 animal kingdom in a line according to rank from the lowest to 

 the highest, and then cut that line into as many or as few 

 pieces as you please, and cut it where you please, and it is 

 evident that the classes thus formed would be heterogeneous 

 and unnatural. Radiates, mollusks, and articulates would be 

 mingled confusedly with each other, and, doubtless, even with 

 the vertebrata. And even though a classification be based 

 upon rank, it would not make the two synonymous. Classifi. 

 cation would then simply be founded on rank as it is com- 

 monly founded on structure. This distinction between grada- 

 tion and classification does not seem to have been apprehended 

 in its full force. A true classification must he based upon differ- 

 ences in hind^ a true gradation upon differences in degree^ dif- 

 ferences in kind being reduced to a common standard by 

 some system of equivalents. In establishing this system of 

 equivalents lies the great difficulty of gi'adation. The mani- 

 fest problem when we attempt to determine rank by speciali- 

 zation or otherwise in the various organs, lies in determining 

 the comparative value of such indications. How much will a 

 modification in locomotion weigh in the scale of rank when set 

 over against a modification in respiration f How much a men- 

 tal characteristic compared with a physical adaptation ? How 

 much of superior specialization in a limb will counterbalance 

 a given amount of intellectual^ m.oral or social development f It 

 is true, this difficulty arises to a certain extent when we take 

 for our basis the nervous system ; yet, I think, here nature has 



