92 THE LIVING WORLD. 



diminution in the abundance of reptiles, though 

 they were still the predominant type. Birds more 

 closely allied to our modern birds began to appear. 



CENOZOIC. 



The Tertiary (9) began the modern era. Modern 

 birds were found in abundance, and the reptiles 

 lost their prestige. True mammals (in distinction 

 from the marsupials) made their appearance either 

 at this time or slightly earlier. During the Tertiary 

 they rapidly developed into the orders which fill the 

 world to-day, and almost immediately became the 

 predominant type. 



The Quart ernary (10), the age of man, brings us to 

 the present time. 



These long periods were of immense duration, but 

 we have no means of determining even approximately 

 how long they lasted. Nor do they by any means 

 represent the whole of geological time. The geo- 

 logical history was doubtless a continuous one, and 

 if we had in our possession the whole of the history 

 we should hardly have been able to divide it into 

 ages. The periods outlined above seem very distinct 

 from each other because they are separated by lost 

 periods of which no record remains to us. Between 

 each of the periods above mentioned there is a break 

 in our record representing the periods during which 

 most important changes took place in the history 

 of life, but of which we can never obtain any knowl- 

 edge. We do not even know whether the lost 

 periods were longer or shorter than those of which 

 we have record, but, judging from the amount of 



