CHAP. ix. THE GIGANTIC HOLOPTYCHIUS. 109 

 /proof which they furnish has convinced me that the 



I theory 



of a gradual progression in size, from the earlier 

 to the later Palaeozoic formations, though based originally 

 on no inconsiderable amount of negative evidence, must 

 be permitted to drop." 



He afterwards refers to the comparatively recent 

 discovery of a gigantic Holoptychius in the Lower Old 

 Red Sandstone of Thurso by Mr. Robert Dick of that 

 place. " It bears shrewdly," he says, " against the line 

 of statement in the text of the book, and it serves to 

 show how large an amount of negative evidence may be 

 dissipated by a single positive fact, and to inculcate on 

 the geologist the necessity of cautious induction."* 



* Hugh Miller's Old Red Sandstone, p. 176. Ed. 1875. 

 6* 



