61:0.1 The era of mammals extends from the times of the origin of placental mammals to the end of the ice age, covering a little less than fifty million years.
61:0.2 During this Cenozoic age the world's landscape presented an attractive appearance—rolling hills, broad valleys, wide rivers, and great forests. Twice during this sector of time the Panama Isthmus went up and down; three times the Bering Strait land bridge did the same. The animal types were both many and varied. The trees swarmed with birds, and the whole world was an animal paradise, notwithstanding the incessant struggle of the evolving animal species for supremacy. 61:0.3 The accumulated deposits of the five periods of this fifty-million-year era contain the fossil records of the successive mammalian dynasties and lead right up through the times of the actual appearance of man himself.