In the _Eocene_ period there was a replacement of the small-brained
archaic mammals by big-brained modernised types, and with this must be
associated the covering of the earth with a garment of grass and dry
pasture. Marshes were replaced by meadows and browsing by grazing
mammals. In the spreading meadows an opportunity was also offered for a
richer evolution of insects and birds.
During the _Oligocene_ the elevation of the land continued, the climate
became much less moist, and the grazing herds extended their range.
The _Miocene_ was the mammalian Golden Age and there were crowning
examples of what Osborn calls "adaptive radiation." That is to say,
mammals, like the reptiles before them, conquer every haunt of life.
There are flying bats, volplaning parachutists, climbers in trees like
sloths and squirrels, quickly moving hoofed mammals, burrowers like the
moles, freshwater mammals, like duckmole and beaver, shore-frequenting
seals and manatees, and open-sea cetaceans, some of which dive far more
than full fathoms five. It is important to realise the perennial
tendency of animals to conquer every corner and to fill every niche of
opportunity, and to notice that this has been done by successive sets of
animals in succeeding ages. _Most notably the mammals repeat all the
experiments of reptiles on a higher turn of the spiral._ Thus arises
what is called convergence, the superficial resemblance of unrelated
types, like whales and fishes, the resemblance being due to the fact
that the different types are similarly adapted to similar conditions of
life. Professor H. F. Osborn points out that mammals may seek any one of
the twelve different habitat-zones, and that in each of these there may
be six quite different kinds of food. Living creatures penetrate
everywhere like the overflowing waters of a great river in flood.
§ 3
The _Pliocene_ period was a more strenuous time, with less genial
climatic conditions, and with more intense competition. Old land bridges
were broken and new ones made, and the geographical distribution
underwent great changes. Professor R. S. Lull describes the _Pliocene_
as "a period of great unrest." "Many migrations occurred the world over,
new competitions arose, and the weaker stocks began to show the effects
of the strenuous life. One momentous event seems to have occurred in the
Pliocene, and that was the transformation of the precursor of humanity
into man--the culmination of the highest line of evolution."
The _Pleistocene_ period was a time of sifting. There was a continued
elevation of the continental masses, and Ice Ages set in, relieved by
less severe interglacial times when the ice-sheets retreated northwards
for a time. Many types, like the mammoth, the woolly rhinoceros, the
sabre-toothed tiger, the cave-lion, and the cave-bear, became extinct.
Others which formerly had a wide range became restricted to the Far
North or were left isolated here and there on the high mountains, like
the Snow Mouse, which now occurs on isolated Alpine heights above the
snow-line. Perhaps it was during this period that many birds of the
Northern Hemisphere learned to evade the winter by the sublime device of
migration.
Looking backwards we may quote Professor Schuchert again:
"The lands in the Cenozoic began to bloom with more and more
flowering plants and grand hardwood forests, the atmosphere is
scented with sweet odours, a vast crowd of new kinds of insects
appear, and the places of the once dominant reptiles of the lands
and seas are taken by the mammals. Out of these struggles there
rises a greater intelligence, seen in nearly all of the mammal
stocks, but particularly in one, the monkey-ape-man. Brute man
appears on the scene with the introduction of the last glacial
climate, a most trying time for all things endowed with life, and
finally there results the dominance of reasoning man over all his
brute associates."
In man and human society the story of evolution has its climax.