THE EVOLUTION OF THE MAMMALIA. 39 
The passage from the Mesozoic to the Tertiary was marked 
by widespread and very important changes in the physical geog- 
raphy of the northern hemisphere and by an extraordinary change 
in the life of the earth. Vegetation had attained almost its mod- 
ern state, but the geographical distribution of plants has been 
greatly altered since that time, owing to the great changes of 
climate which have occurred. The huge and bizarre reptiles of 
the Mesozoic had all disappeared, while the mammals come to 
the front in an astonishing outburst, as it may fairly be called. 
Henceforth the mammals were to be the dominating type, taking 
the place of the dethroned reptiles. 
Much of mammalian history has been preserved in the fresh- 
water deposits of Tertiary age in various parts of the world, but 
in no region yet known with such fullness as in the western part 
of the United States, where are found deposits covering almost 
the whole of Tertiary time. These great rock masses, of differ- 
ent dates, cover thousands of square miles, and were laid down 
in various ways. Some were accumulated in lake-basins, others 
were spread by sluggish streams over their flood-plains, others 
may have been heaped up by the winds in semi-deserts. The 
rocks thus accumulated entombed the bones of innumerable ani- 
mals, the fossil remains of which are not only extraordinarily 
abundant, but are preserved in a degree of completeness found in 
very few other parts of the world. Many entire skeletons have 
already been recovered, and exploration is continually bringing 
others to light. These Tertiary fresh-water rocks are almost all 
soft and some of them quite loose and incoherent; in the arid 
regions where they occur they are little or not at all protected by 
vegetation, and are rapidly carved by the atmosphere into those 
areas of fantastic and weird topography known as the "bad 
lands." 
The principal horizons of the Western Tertiary deposits are 
grouped in the following table, and in the order of their succes- 
sion, the oldest at the bottom, and the newest at the top. It will 
be necessary to refer constantly to these formations by name in 
tracing out the history of the mammalian groups. 
