CHAPTER XIII 
SOME EARLY MAMMALS AND THE STORY OF THE HORSE 
** Geology, in the magnitude and sublimity of the objects of which it treats, 
ranks next to astronomy in the scale of the sciences.’—Sir Joun F. W. 
HERSCHEL. 
It has often been said that the Primary era was an “Age of 
Fishes,” the Secondary era an “Age of Reptiles,’ and the 
Tertiary an “Age of Mammals,” There is, however, a danger 
lest beginners should be deceived by broad statements of this 
kind, which must not be pressed too far. All that is meant in 
this case is, that, first of all, fishes were the dominant type; then 
reptiles, and then mammals. 
Fishes abounded in the waters of the Primary or Paleozoic 
era, While air-breathing amphibians appeared towards its close; 
reptiles flourished vigorously all through Secondary or Mesozoic 
times: and now we are about to show that a marvellous outburst 
of mammalian life appears to have taken place very early in 
the Tertiary or Cainozoic era. All this is in accordance with the 
“Law of Progress” throughout past times, and strongly confirms 
the theory of Evolution (see pp. 84, 106, 125, and 264). 
The student of geological history soon discovers that mammals 
of some kind, or kinds, did exist throughout the Secondary era— 
although, judging from their imperfect remains, they must have 
been of a low type.'! But the great advance in our knowledge of 
* Perhaps some of the jaw-bones from Triassic and Jurassic strata represent 
creatures as low down in the scale as the Monotremes, represented at the 
present day by the Duck-bill Platypus and the Echidna of Australia, 
