434 ON THE METHOD OF PAL.^iONTOLOGY 



sively on physiological grounds alone from any part of a living being 

 to the whole. 



I by no means assert that Cuvier, in enunciating the proposition 

 quoted above, meant to exclude all but physiological considerations 

 so completely as the words appear to indicate. On the contrary, his 

 practice, no less than other passages of the remarkable essay from 

 which that citation is taken, shows clearly that no man more fully 

 understood the value of morphology. Nevertheless the words of the 

 proposition are distinct enough to justify those who, guided more by 

 authority than by right reason, have denominated it Cuvier's law of 

 correlation, and, ambiguously supported by Cuvier's phraseology 

 elsewhere, have imagined the principle which it involves to have been 

 his guide in paljeontological research. 



A simple illustration or two, however, will show that the laws of 

 physiological correlation alone are wholly incompetent to furnish such 

 guidance. Suppose I find the jaw of a vertebrate animal with sharp 

 cutting teeth imbedded in it, how far will physiology help me to 

 determine the precise nature of the animal to which it belonged ? 

 The sharpness of the teeth may lead me to guess that they were used 

 for cutting some soft substance. The shape of the articular condyle 

 and that of the processes for muscular attachment may equally render 

 probable the direction and force of its ordinary movements ; but as to 

 the rest of the organism, whether the teeth were for cutting up fish, 

 flesh, fowl, or carrion, whether the creature itself was piscine or 

 reptilian or mammalian, — on all these points no amount of mere 

 physiological reasoning will help me. Nay, how do I know it is a 

 vertebrate jaw at all ; that it is a vertebrate bone and tooth sub- 

 stance ? For anything physiology teaches me to the contrary, 

 Invertebrate animals might develope osseous and dentinal tissue, 

 and might possess appendages having the form of vertebrate jaws. 



Every naturalist knows that Invertebrate animals do not thus mimic 

 the Vertebrata, and he believes that they never have and never will 

 do so ; but his confidence is based, not on. any physiological reasoning 

 as to the impossibility of such a proceeding, but on his simple 

 experience that it never does occur. He rests not on a deduction 

 from the laws of physiological correlation, but on the morphological 

 law that no Invertebrate animal ever possesses an organ having the 

 form and structure displayed by the jaw in question. And this law is 

 an empirical one ; no further reason for it can be given than for the 

 law of gravitation. The whole object of morphology is to ascertain 

 what structural peculiarities invariably coexist with one another : why 

 these structural peculiarities coexist is a question with which it does 



