THE VOYAGE OF EVOLUTION 115 



Australia. Moreover, the early mammals were but a feeble folk. 

 The reptiles were the great lords of creation, while the mammals 

 were tiny little beasts skulking in out-of-the-way corners. Not 

 for long ages did they rise to dominance in the lowland regions 

 where the great reptiles lorded it. Among the hills, however, or 

 elsewhere in safe retreats they were slowly evolving two of their 

 vital characteristics, namely, a uniform temperature and a large 

 brain. 



Once more we must skip millions of years. The mammals have 

 grown in size and variety until they range from the mouse to the 

 mammoth. They have ousted the reptiles from the best parts of 

 the earth. They have taken to the air with the wings of the bat, 

 they have gone back to the sea with the whale, they have learned 

 to run like the antelope, to burrow like the mole, and to climb 

 trees like the squirrel. Their limbs have become hoofs, claws, 

 wings, flippers, and hands. The Age of Mammals has come to its 

 epiphany. Then, as in Permian times, there once more comes a 

 widespread period of climatic stress, the last Glacial Period. A 

 new element enters into evolution, for at last man appears, and 

 intelligence becomes dominant. 



Before we consider man let us pause for a moment to estimate 

 how great was the change in life wrought by the Glacial Period. 

 For this purpose I have taken the table of mammalian genera in 

 Osborn's "Age of Mammals." It includes every known genus 

 whether living or dead. From this table I have ascertained the 

 number of genera which were alive during the geological periods 

 preceding the Glacial Period. The number of genera has been 

 counted for Europe and North America separately. I have also 

 ascertained the total number extinguished in each period and the 

 percentage that were extinguished on an average during each 

 hundred thousand years. The method by which this has been done 

 is explained in the Appendix. 



The results of this tabulation of mammals are shown graphi- 

 cally in Figure 22. There the names of the geological periods 



