【单选题】

The history of mammals dates back at least

to Triassic time.Development was retarded, how-
ever, until the sudden acceleration of evolutional
Line change that occurred in the oldest Paleocene. This
(5) led inEocene time to increase in average size,
larger mental capacity, and special adaptations for
different modes of life. In the OligoceneEpoch,
there was further improvement, with appearance
of some new lines and extinction of others.
(10) Miocene and Pliocene time was marked by culmi-
nation of several groups and continued approach
toward modern characters. The peak of the career
of mammals in variety and average large size was
attained in the Miocene.
(15) The adaptation of mammals to almost all pos-
sible modes of life parallels that of the reptiles in
Mesozoic time, and except for greater intelligence,
the mammals do not seem to have done much bet-
ter than corresponding reptilian forms. The bat is
(20) doubtless a better flying animal than the pterosaur,
but the dolphin and whale are hardly more fishlike
than the ichthyosaur. Many swift-running mam-
mals of the plains, like the horse and the antelope,
must excel any of the dinosaurs. The tyrannosaur
(25) was a more ponderous and powerful carnivore than
any flesh-eating mammal, but the lion or tiger is
probably a more efficient and dangerous beast of
prey because of a superior brain. The significant
point to observe is that different branches of the
(30) mammals gradually fitted themselves for all sorts
of life, grazing on the plains and able to run swiftly
(horse, deer, bison), living in rivers and swamps
(hippopotamus, beaver), dwelling in trees (sloth,
monkey), digging underground (mole, rodent),
(35) feeding on flesh in the forest (tiger) and on the plain
(wolf), swimming in the sea (dolphin, whale, seal),
and flying in the air (bat). Man is able by mechani-
cal means to conquer the physical world and to
adapt himself to almost any set of conditions.
(40) This adaptation produces gradual changes of
form and structure. It is biologically characteristic
of the youthful, plastic stage of a group.Early in its
career, an animal assemblage seems to possess
capacity for change, which, as the unit becomes
(45) old and fixed, disappears. The generalized types of
organisms retain longest the ability to make adjust-
ments when required, and it is from them that new,
fecund stocks take origin—certainly not from any
specialized end products. So, in the mammals, we
(50) witness the birth, plastic spread in many directions,
increasing specialization, and in some branches,
the extinction, which we have learned from obser-
vation of the geologic record of life is a characteris-
tic of the evolution of life.
It can be inferred from the passage that the evidence the author uses in discussing the life of past time periodsA．was developed byCharlesDarwin
B．was uncovered by the author
C．has been negated by more recent evidence
D．was never definitely established
E、is based on fossil remains