xxx PREFACE. 



in the metacarpus and metatarsus, or more numerous tarsal bones ; 

 or a more distinct and better developed fibula ; or a concurrence 

 of all these modifications. It is impossible to assign a reason for 

 these relations ; but, in proof that it is not an affair of chance, 

 we find that whenever a bisulcate animal shows in its dentition 

 any tendency to approach the non-ruminant ungulates, it also 

 manifests a similar tendency in the conformation of its feet.' After 

 citing similar instances of such constant relations, Cuvier again 

 declares that the palaeontologist ' must avail himself of the method 

 of observation' as a supplementary instrument when the reason or 

 law of such relations is undiscovered ; and that he is most suc- 

 cessful in the reconstruction of a whole from a part, who applies 

 to the task ' efficacious comparison,' guided by 'tact (adresse) in 

 discerning- likeness.' ' 



As we descend in the scale of life from the grade illustrative 

 of ' Cuvier's LaAv,' the method of empirical observation becomes 

 more and more essential, the tact with which it is applied being, 

 however, in the ratio of the discernment of the correlations of 

 structures. The results of the combined methods of interpreting 

 fossil remains are leading to views of life transcending the gains 

 to zoology as a record of well-classed species, or to physiology as 

 illustrative of final purpose. A progress from more generalised 

 to more specialised structures, analogous to that exemplified in 

 existing grades of animal life and in successive phases of individual 

 development, is appreciable in the series of species which have 

 succeeded one another upon our planet. 



Certain structures Avhich are transitory or rudimental in exist- 

 ing species are persistent and developed in extinct. 



The caudal vertebras are laid down in a gradually decreasing 

 series of cartilaginous nuclei, in the embryo of modern bony fishes ; 

 but in the course of ossification they become massed and blended 

 together to form the base of a vertically extended symmetrical 

 tail-fin. In all palaeozoic fishes the initial embryo-state persists, 

 and the tail-fin, through the length of the upper lobe retaining the 



1 Op. i-it. p. 52. 



