oman
patricians--and of the Ammonites may very well have been caused by
physical and chemical changes in the sea-water. We know from experiment
that such changes have a disturbing influence, especially on the
development of eggs and larvae; and we know from the geological record
that such changes occurred in the periods when the Trilobites and
Ammonites perished. In fine, the vast majority of extinct races passed
through no "convulsions" whatever. We may conclude that races do not
die; they are killed.

The extinction of these races of the early Condylarthra, and the
survival of those races whose descendants share the earth with us
to-day, are quite intelligible. The hand of natural selection lay heavy
on the Tertiary herbivores. Apart from overpopulation, forcing groups
to adapt themselves to different regions and diets, and apart from the
geological disturbances and climatic changes which occurred in nearly
every period, the shadow of the advancing carnivores was upon them.
Primitive but formidable tigers, wolves, and hyenas were multiplying,
and a great selective struggle set in. Some groups shrank from the
battle by burrowing underground like the rabbit; some, like the squirrel
or the ape, took refuge in the trees; some, like the whale and seal,
returned to the water; some shrank into armour, like the armadillo, or
behind fences of spines, like the hedgehog; some, like the bat, escaped
into the air. Social life also was probably developed at this time, and
the great herds had their sentinels and leaders. But the most useful
qualities of the large vegetarians, which lived on grass and leaf, were
acuteness of perception to see the danger, and speed of limb to escape
it. In other words, increase of brain and sense-power and increase of
speed were the primary requisites. The clumsy early Condylarthra failed
to meet the tests, and perished; the other branches of the race were
more plastic, and, under the pressure of a formidable enemy, were
gradually moulded into the horse, the deer, the ox, the antelope, and
the elephant.

We can follow the evolution of our mammals of this branch most easily
by studying the modification of the feet and limbs. In a running
attitude--the experiment may be tried--the weight of the body is shifted
from the flat sole of the foot, and thrown upon the toes, especially the
central toes. This indicates the line of development of the Ungulates
(hoofed animals) in the struggle of the Tertiary Er