CUVIER. 159 



palaeontologist is usually called upon to frame his con- 

 clusions on a fragmentary specimen. He may have only 

 a single bone or tooth ; or he may have a number of 

 detached bones. Only rarely does he find a complete 

 skeleton, or meet with the bones still in their proper 

 places and connections. 



This inherent difficulty in all palaeontological investiga- 

 tion was solved by the establishment by Cuvier of the 

 famous law of ' the correlation of organs.' Cuvier showed 

 that certain organs or structures in animals are only 

 found in association with one another; so that if one 

 of these correlated organs be found to be present, then we 

 may be sure that the others will also be there. In some 

 cases, this correlation or association of particular organs is 

 based upon an obvious physiological connection. Thus, 

 thin-walled hollow bones are associated with a peculiar 

 form of lung, in which the greater air-passages (bronchi) 

 do not end within the lung itself, but become connected at 

 the surface of the lung with membranous receptacles or 

 air-sacs distributed in different parts of the body. Again, 

 the peculiar form of toe-bone which is adapted for the 

 carrying of a hoof is correlated with such other modifica- 

 tions of the bones of the limb as are needed to secure the 

 absence of rotation in the bones, and to insure the fitness 

 of the leg for its special function of supporting the weight 

 of the body. In very many cases, however, no physio- 

 logical explanation can be given as to the association or 

 correlation of particular organs. Thus, all animals in 

 which the skull is jointed to the backbone by a double 

 articulation, and in which the two halves of the lower jaw 

 are composed each of a single piece, have at the same 



