186 
52. And this brings me to the Second break in the con- 
tinuity of mammalian life. 
53. Professor Marsh says that the native horses of America 
were all extinct, and that at a very early period. The pre- 
sumed palaeolithic man in America had no horses. 
POST GLACIAL GRAVELS. 
PLEISTOCENE. 
GLACIAL BEDS. 
PLIOCENE. 
MIOCENE. 
EOCENE. 
hH 
CHALK. 
CRETACEOUS. 
UPPER GREENSAND. 
GAULT. 
LOWER GREENSAND. 
PURBECK BEDS. 
OOLITE. 
STONESFIELD SLATE. 
RHAETIC BEDS. 
LIAS. 
TRIAS. 
54. Professor Marsh does not mention the glacial conditions as 
the cause of that extinction. He calls the extinction a mystery. 
To Principal Dawson it was no mystery ; for you will remember 
that he said the land went under water 4,000 feet in depth, 
and that the glacial age proved fatal to a large proportion of 
the land-life. If so, does not that solve the mystery ? 
55. But, if the horse did survive that period, we come now 
to a still greater diflS.culty, a difficulty which is shared by all 
the great mammalian pachyderms of the Eocene period. 
56. Professor Marsh, in his Introduction and Succession of 
Vertebrate Life in America (an address delivered before the 
American Association for the Advancement of Science, August, 
