IX .1 ON A PIECE OF CHALK. 171 



unless his antiquity dates infinitely further back than we at 

 present surmise. If we could be carried back into those times, 

 we should be as one suddenly set down in Australia before it 

 was colonized. We should see mammals, birds, reptiles, fishes, 

 insects, snails, and the like, clearly 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 between 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. 



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

 ancient and the modern inhabitants of the world are most com 

 pletely 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 ichthyosaurus, 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 



