e 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.

Sec. 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 occu