10 THE STGRY OF FISH LIFE. 



still more primitive forms, whose structure did not 

 permit of preservation, lacking hard parts such 

 as bones or scales. The remains of certain other 

 early types leave us yet in doubt on very important 

 points, such as whether the mouth had jaws, 

 what was the position of the eyes, and so on. 

 Answers to many of these questions may, how- 

 ever, yet come to hand, for the examination of rock 

 systems of the world is by no means exhausted yet. 



These remains, then, we know as fossils, and 

 the hardened mud in which they are embedded 

 we call rock. How these rocks came to be does 

 not concern us here. Those who would know 

 more on this head should read Prof. Seeley's little 

 book, the " Story of the Earth," published in 

 this series. Suffice it to say, that the various 

 kinds of mud with their peculiar fossils repre- 

 sent different periods of time of great duration. 

 These periods we shall refer to here under their 

 scientific names, as "Devonian," "Carboniferous " 

 or " Cretaceous," and so on as the case may be. 

 We shall find that as we proceed from the oldest 

 to the newest of these formations that the fossils 

 therein will increase in number, variety and 

 complexity of structure. This increased com- 

 plexity has resulted from the gradual modification 

 of simple types as they become more and more 

 perfectly adapted to their environment. 



Those of us who can spell out more or less 

 connected portions of these riddles have gathered 

 one fact of prime interest. We cannot fail to be 

 impressed with the conviction that Nature has 

 pressed the plastic mystery which we call Life 

 into many moulds, and many creatures into the 



