36 rrWKH 



well-developed eyes arm cars, claws and teeth. Further, you 

 know from experience that such and such definitely modified 

 organs are invariably found with the carnivorous habit, 

 carnassial teeth, for example, and reduced clavicles. From 

 a "carnivorous" alimentary canal, then, you can infer with 

 certainty that the animal possessed carnassial teeth and the 

 other structural peculiarities of carnivorous animals, e.g., 

 the peculiar coronoid process of the mandible. From the 

 carnassial tooth you can infer the reduced clavicle, and so 

 on. " In a word, the form of the tooth implies the form of 

 the condyle ; that of the shoulder blade that of the claws, 

 just as the equation of a curve implies all its properties." 1 



Similarly the great respiratory power of birds is corre- 

 lated with their great muscular strength, and renders 

 necessary great digestive powers. Hence the correlated 

 structure of lungs, muscles and their attachments, and 

 alimentary canal, in birds. 



Not only do systems of organs, by being adjusted to 

 special modifications of function, influence one another, but 

 so also do parts of the same organ. This is noticeably the 

 case with the skeleton, where hardly a facet can vary without 

 the others varying proportionately, so that from one bone 

 you can up to a certain point deduce all the rest. 



We deduce the necessity, the constancy, of these co-exis- 

 tences of organs from the observed reciprocal influence of 

 their functions. That being established, we can argue from 

 observed constancy of relation between two organs an action 

 of one upon the other, and so be led to a discovery of their 

 functions. But even if we do not discover the functional 

 interdependencies of the parts, we can use the established 

 fact of the constant co-existence of two parts as proof of a 

 functional correlation between them. 



('orrelation is either a rational or an empirical principle, 

 according as we know or do not know the interdependence of 

 function of which it is the expression. Fven when we apply 

 the rational principle of correlation it would be useless in our 

 hands if we had not extensive empirical knowledge ; when 

 we use an empirical rule of correlation we depend entirely 

 upon observation. "There area great many cases," writes 

 1 Recherches sur les Ossancns Fossil es, i., p. o, 1812. 



