OF CREATION. 47 



should afterwards abound. Although, however, these 

 fishes were introduced towards the close but before 

 the termination of the period of the Invertebrata, it is 

 important to remember that almost all the great 

 natural divisions of the Invertebrata began at once 

 and together to perform their work on earth ; so that 

 there is no appearance of any regular order of progres- 

 sion by which the encrinite succeeded the coral polyp, 

 the trilobite the encrinite, the terebratula the tri- 

 lobite, or the orthoceratite the brachiopod. All these 

 seem to have been truly contemporaneous, and they 

 were doubtless introduced as the group best fitted to 

 perform the functions of their existence during the 

 conditions, whatever they may have been, under which 

 the world existed in their time. And so little in many 

 points do the differences of their organization seem to 

 require important changes in the temperature and 

 atmospheric condition of the earth, so similar are the 

 living species most nearly allied to them in all pecu- 

 liarities of which we can fairly judge, that, however 

 we may be inclined to conjecture and speculate on 

 the probability of a higher and more uniform tempe- 

 rature, a more widely extended sea receiving similar 

 deposits and containing similar species, or an atmo- 

 sphere more highly charged with carbonic acid gas 

 than at present, these speculations must be kept 

 within bounds, since the facts justify no more than the 

 admission of their bare possibility. 



The mere absence of certain groups or of certain 

 species and genera afterwards common, is not the 

 only point on which the naturalist dwells in con- 

 sidering the possible conditions of the ancient sea. 

 It is much more important, and much more interest- 



