156 THE STORY OF THE EARTH AND MAN. 



To return to our Trilobites : one of the most re. 

 markable points in their history is their appearance 

 in full force in the Primordial. In these rocks we 

 have some of the largest in size some species of 

 Paradoxides being nearly two feet long, and some of 

 the very smallest. We have some with the most nu- 

 merous joints, others with the fewest; some with very 

 large tails, others with very small ; some with no 

 ornamentation, others very ornate; some with large 

 eyes, others with none that have been made out, 

 though it is scarcely probable that they were wholly 

 blind. They increased in numbers and variety through 

 the Silurian and Devonian, and then suddenly drop 

 off at the end of the Lower Carboniferous. Through- 

 out their whole term of existence t\ey kept rigidly 

 to that type of the mud-plough which the king-crab 

 still retains, and which renders the anterior extrem- 

 ity so different from that of the ordinary Crustacea. 

 They constitute one of the few cases in which we seem 

 to see before us the whole history of an animal type ; 

 and the more we look into that history, the more do 

 we wonder at their inscrutable introduction, the unity 

 and variety mingled in their progress, and their 

 strange and apparently untimely end. I have already 

 referred (page 95) to tho use which Barrande makes 

 of this as an argument against theories of evolution; 

 but must refer to his work for the details. 



One word more I must say before leaving their 

 graves. I have reason to believe that they were 

 not only the diggers of the burrows, and of the 



