OF CREATION. 261 



have left undoubted marks of their existence and 

 wide distribution. 



This method of substitution, by which, at different 

 times and in various places, animals and vegetables 

 of very different organization have replaced one an- 

 other, is found by geological observations to have been 

 universally adopted in nature. Large groups may be 

 observed performing their appointed task at one pe- 

 riod or in one place, and these are represented by 

 other species at another time or at some distant 

 spot, the representative species exhibiting some ana- 

 logous peculiarity, either of structure or habit, which 

 fits it for that similar function. 



This important fact is, however, not made known 

 to us exclusively by geological researches, but also by 

 investigations into existing nature ; and these latter, 

 when combined with the former, become more dis- 

 tinctly recognised, and by their manifest relation to 

 long periods of time as well as extensive tracts in 

 space, exhibit their true importance with reference 

 to all nature. 



Viewed as one epoch, the secondary rocks present 

 to us a well-marked and very distinct group of ani- 

 mals and vegetables, from the careful study of which 

 we may arrive at some probable conclusion with 

 regard to the condition of the earth and seas during 

 the period of deposit. We thus learn to consider the 

 secondary period as indicating a condition very differ- 

 ent from that of the earlier epoch, and as cut off from 

 it absolutely by the introduction of new forms of or- 

 ganic existence. Its great characteristic undoubtedly 

 appears to be the introduction of reptiles, or rather the 

 presence of reptiles in such vast variety of form, and 



