62 EXPLANATIONS. 



tions, upon the idea that a formation represents 

 one point in time. A formation, in reality, repre- 

 sents many years, or rather ages. Such expres- 

 sions as that simple and complex plants occm' 

 together in the carboniferous formation, or even 

 (shall we say) in its first fossil bands, are vague 

 expressions ; perhaps, conveying an idea substan- 

 tially false. There is no such precision in the as- 

 certained relations of fossils to particular strata 

 as to entitle any one to say that the simple and 

 complex plants of this formation are rigidly con- 

 temporaneous. They may have followed each 

 other within the space of half a century in a par- 

 ticular region, and yet been preserved in but one 

 stratum, or little group of strata. The actual appear- 

 ances of the carboniferous formation thus, perhaps, 

 allow full time for a progressive advance in parti- 

 cular regions, from the fleshy luxuriant plants of 

 the marsh and low sea-margin, to the robust tree 

 of the more elevated regions. We must remember, 

 too, that the vegetation of the carbonigenous era, 

 even if we take it back to include the conifer said to 

 have lately been found in the Old Red of Cromarty, 

 or the fern leaf of the Silurians, was preceded by 

 unequivocally simple plants in the fucoids. Start- 



