74 THE STORY OP THE EAETH AND MAN. 



currence, I am inclined to believe that in their 

 younger stages they were attached, but by a very 

 slender thread; that at a more advanced stage they 

 became free, and acquiring a central membranous 

 disk filled with air, floated by means of this at the 

 surface, their long branches trailing in the waters 

 below. They would thus be, with reference to their 

 mode of life, though not to the details of their 

 structure, prototypes of the modern Portuguese man- 

 of-war, which now drifts so gaily over the surface of 

 the warmer seas. I have represented them in this 

 attitude ; but in case I should be mistaken, the reader 

 may imagine it possible that they may be adhering 

 to the lower surface of floating tangle. The head- 

 quarters of the Graptolites seem to be in the upper 

 part of the Cambrian, and in the Siluro- Cambrian^ 

 and they are widely distributed in Europe, in 

 America, and in Australia. This very wide distribu- 

 tion of the species is probably connected with their 

 floating and oceanic habits. 



Lastly, just as the Silurian period was passing 

 away, we find a new thing in the earth vertebrate 

 animals, represented by several species of shark-like 

 fishes, which came in here as forerunners of the 

 dynasty of the vertebrates, which from that day to 

 this have been the masters of the world. These 

 earliest vertebrates are especially interesting as the 

 first known examples of a plan of structure which 

 culminates only in man himself. They appear to 

 have had cartilaginous skeletons ; and in this an<3 



