PALEONTOLOGY. 209 



Each of the periods into which the palaeontologist divides 

 the history of the earth, is characterised by a distinct suite 

 of organic remains. Thus the mollusca, Crustacea, and fishes 

 of the palaeozoic rocks, differ from those of the secondary 

 series. The fishes and reptiles of the oolitic and cretaceous 

 periods are different from those of the tertiary. The spe- 

 ciality of fossils is not limited to families and genera, but 

 extends even to the species of the animal series, so that a 

 practised eye can at once determine to which stage of any 

 system of rocks a given suite of fossil remains belong. 



From the period this great fact was first enunciated, down 

 to the present time, it has received from each succeeding 

 palaeontologist some important additions ; but by none has 

 its details been developed with so much patient research, 

 and illustrated with such scientific precision, as by Alcide 

 D'Orbigny, the learned and accurate author of the " Pale- 

 on tologie Pran9aise," to whose works we must refer the 

 student for ample details on this most interesting branch of 

 inquiry. 



The association of certain species of fossil animals in each 

 chapter, so to speak, of the history of the earth's crust, has 

 demonstrated the existence of a series of distinct faunas, 

 from which the palaeontologist has inferred the existence of 

 certain laws which appear to have governed the same. His 

 generalisations, it is true, are not unexceptionable ; still 

 they have contributed to the progress of science, and have 

 called the attention of naturalists to -many interesting and 

 important questions that have arisen in the study of organic 

 remains. 



The five principal laws which have been established 

 relative to the question of successive faunas, have been well 

 explained by Professor Pictet, in his valuable work,* to 

 which we refer for farther details. 



FIRST LAW. The species of animals of one geological epoch 

 lived neither before nor after that epoch, each formation has 

 its own fossil species, and the same species is never found in 

 two strata of a different age. 



Although the generality of this law is not admitted to the 



* Traite Elementaire de Paleontologie, torn. i.p. 58. 



p 



