I ON A PIECE OF CHALK 31 



recognizable as such, and yet not one of them 

 would be just the same as those with which we 

 are familiar, and many would be extremely 

 different. 



From that time to the present, the population 

 of the world has undergone slow and gradual, but 

 incessant, changes. There has been no grand 

 catastrophe no destroyer has swept away the 

 forms of life of one period, and replaced them by 

 a totally new creation : but one species has 

 vanished and another has taken its place ; 

 creatures of one type of structure have diminished, 

 those of another have increased, as time has 

 passed on. And thus, while the differences be- 

 tween the living creatures of the time before the 

 chalk and those of the present day appear 

 startling, if placed side by side, we are led from 

 one to the other by the most gradual progress, if 

 we follow the course of Nature through the 

 whole series of those relics of her operations which 

 she has left behind. It is by the population of 

 the chalk sea that the ancient and the modern 

 inhabitants of the world are most completely con- 

 nected. The groups which are dying out flourish, 

 side by side, with the groups which are now the 

 dominant forms of life. Thus the chalk contains 

 remains of those strange flying and swimming 

 reptiles, the pterodactyl, the ichthyosaurus, and 

 the plesiosaurus, which are found in no later 

 deposits, but abounded in preceding ages. The 





