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 
