No. 381.]} A HALF-CENTURY OF EVOLUTION. 669 
of the wonderful changes, both geological and zodlogical, which 
occurred in western America during the Tertiary. They are 
now familiar to every one. The geological changes were very 
great and widespread, as shown by the elevation of the land at 
the close of the Miocene. Fragments of the Cretaceous sea 
bottom, with horizontal strata, occur in the Rocky Mountains 
at a point about 10,000 feet above the sea. The inland Creta- 
ceous sea was drained off, and replaced by a series of fresh- 
water lakes, beginning with the Puerco, or the lowest Eocene, 
and ending with the Pliocene lakes. 
The most salient biological features of the Tertiary are the 
apparently sudden appearance all over the world of placental 
mammals, ending, if the deposits are truly Pliocene, with the 
Java Pithecanthropus, and at the beginning of the Quaternary 
with paleolithic man. 
The question here arises as to what retarded the progress in 
the mammalian types, although small, generalized, feeble insect 
eaters had originated certainly in the Triassic, and probably as 
early as the end of the Permian. We can only account for it 
by the unfavorable biological environment, by the apparently 
overwhelming numbers of Mesozoic reptiles, adapted as they 
were for every variety of station and soil, whether on land, in 
the ocean, in the lakes and rivers, and even in the air. 
When the reptiles became partly extinct, a great acceleration 
in the evolution of mammals at once resulted. There were 
now upland grassy plains, bordered by extensive forests, which 
also clothed the highlands, and all the geographical conditions 
so favorable to mammalian life became pronounced after the 
Cretaceous seas were drained off. 
In his admirable essay on The Relation between Base-Level- 
ing and Organic Evolution, which we had not read until after 
planning and writing this address, though following the same 
line of thought, Mr. J. B. Woodworth suggests that mammalian 
life in the Mesozoic was unfavorably affected by the peneplain 
and by reptilian life. 
“The weak marsupials or low mammals, which first appear in this 
country with Dromatherium in the tolerably high relief of the Trias, were 
apparently driven to the uplands by the more puissant and numerous 
