XV111 PREFACE. 



order of geological position, or can be traced ex- 

 clusively to particular strata. To have given 

 that which is, or is thought to be, really known 

 on this subject, would have been of little utility ; 

 as the numerous exceptions, and the chances of 

 error, must have produced a mixture of uncer- 

 tainty and confusion that would have defeated 

 the very objects for which this knowledge is 

 principally valuable. 



Even the mere history of the numerous fossil 

 species, whether of plants or animals, must be 

 the work of time ; and can only be improved as 

 the numbers of those geologists who have culti- 

 vated the study of botany, zoology, and compa- 

 rative anatomy, shall increase. That history 

 must also be completed before it can be ef- 

 fectually connected with the circumstances of 

 their geological position and relations ; nor will 

 this connection be easily formed unless geological 

 knowledge is united in the same individual to an 

 accurate acquaintance with these organic sub- 

 stances. The rapidly increasing number of those 

 who cultivate these departments of natural know- 



