108 PP. REPORT OF PROGRESS. FONTAINE & WHITE. 



brates, and Plants. It seems to ns erroneous to claim ab- 

 solntely, that one of these must be valued more highly 

 than another, for the evidence it affords. We must limit 

 the applicability of the evidences from each class. 



A priori, we might decide as follows : Mollusks, from 

 the simplicity of their structure, and the nature of the 

 medium in which they live, could not be seriously affected 

 by slight changes of the surrounding conditions, and hence, 

 when not interrupted by cataclysmal agencies, their re- 

 mains can only be used to denote general changes, requir- 

 ing long periods of time. They are the hour hand of the 

 palsDontological clock. But we must admit the possibility 

 of the existence of special local causes, which may hasten 

 their changes. The same may also occur to modify the 

 normal character of the Vertebrates and Plants. We must 

 however have positive evidence of their existence. Plants, 

 being more dependent on aerial conditions, and less capable 

 of resistance, should give better data for indicating slighter 

 changes, involving shorter periods. They are the minute 

 hand of our clock. Vertebrates are in structure the most 

 complex of the three. They depend in part on plants, and 

 in part, on aerial conditions, or the arrangement of the land 

 and water. Hence they are the most sensitive time indica- 

 tors, and mark slighter changes requiring shorter intervals. 

 They record the seconds on our clock. Their sensitiveness 

 unfits them for the determination of the longer intervals, 

 which have been founded on the evidences derived from 

 Mollusks or Plants. Relying on them, we would antedate 

 the age of the formation which affords them. 



We may hence consider that so far as we can lay down 

 a general rule for the applicability and relative value of the 

 evidence from the three most important classes of organ- 

 isms, it would be as follows : The evidence of Mollusks 

 should be most weighty in determining long periods ; that 

 of Plants, most important in shorter intervals, and that of 

 Vertebrates in the shortest. Of course we must take into 

 consideration all the forms of any one of these classes. It 

 is no more necessary to take a group of plants, in order to 

 get evidence of value, than it is to study the entire collec- 



