LIFE IN THE PAST 183 



appearance, but of momentous importance on account 

 of their later history. Among these reptiles were a 

 few small creatures perhaps not much bigger than 

 mice or moles. Their teeth were a little more com- 

 plicated and specialized than the teeth of their rep- 

 tilian cousins. Between their scales were small and 

 sparse hairs. Almost nothing but their jaws remain 

 to-day to tell us anything about them. But in this 

 humble little creature of the Mesozoic, utterly insig- 

 nificant beside the tremendous reptiles of the time, we 

 discern the ancestor of the mammals. These were 

 the progenitors of the horses and cows, of the cats 

 and dogs, of the monkeys and apes, of the men of 

 to-day. 



During this chalk period, which forms the last por- 

 tion of the age of reptiles, life for the first time grew 

 to look much as it does to-day. Now, apparently, the 

 cold of winter and the heat of summer followed each 

 other in regular succession. There have been colder 

 and warmer periods at various times in the previous 

 history of the earth, but undoubtedly they were more 

 uniformly cold or uniformly warm than now. Ages 

 were warm, or ages were cold, but now the earth 

 clearly shows the annual alternations of summer and 

 winter, and for the first time clearly shows the bands 

 of climate on the earth which we know as zones. 



In the chalk period this new factor of cold works 



