68 ON A PIECE OF CHALK 



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 di- 

 minished, 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 Na- 

 ture through the whole series of those relics of her 

 operations which she has left behind. 



And it is by the population of the chalk sea that the 

 ancient and the modern inhabitants of the world are 

 most completely connected. 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 ich- 

 thyosaurus, and the plesiosaurus, which are found in no 

 later deposits, but abounded in preceding ages. The 

 chambered shells called ammonites and belemnites, 

 which are so characteristic of the period preceding 

 the cretaceous, in like manner die with it. 



But, amongst these fading remainders of a previous 

 state of things, are some very modern forms of life, 

 looking like Yankee pedlars among a tribe of Red 

 Indians. Crocodiles of modern type appear; bony 

 fishes, many of them very similar to existing species 

 almost supplant the forms of fish which predominate 

 in more ancient seas; and many kinds of living shell- 

 fish first become known to us in the chalk. The vege- 

 tation acquires a modern aspect. A few living animals 

 are not even distinguishable as species, from those 



