i 4 PHYSIOLOGICAL CLASSIFICATION 



upon a profounder analysis of the animated 

 frame, namely, upon the cellular constitution of 

 the body. This second great generalisation, as 

 we have seen above, had already been achieved 

 during the first half of the preceding century, 

 but the fate of the backbone in classification 

 was not sealed until the end. In the light of 

 recent researches we may find a further reason, 

 besides the fallacy of contrasting non-equivalent 

 groups, for discarding the older method. The 

 presence of a backbone is a character which 

 does not pair with the absence of one, but it 

 does pair with the presence of a notochord. 



We may venture to lay down, as a funda- 

 mental principle in morphology, that only those 

 characters pair which are connected together by 

 the bond of substitution. By way of extending 

 this principle one step further whilst reference 

 is being made to the vertebrata, we may add 

 that the presence of a bony endoskeleton is a 

 character which pairs morphologically with a 

 cartilaginous endoskeleton, but not with a horny 

 exoskeleton. On the other hand the vertebrate 

 endoskeleton, in its capacity of affording attach- 

 ment to muscles, can be compared physiologically 

 with an arthropod exoskeleton or a molluscan 

 shell. 



A morphological classification must be physio- 

 logically true, i.e., it must conform to physio- 

 logical laws ; but a physiological classification is 



