On the other hand, the eccentricities of many of the
later Trilobites--the LATEST Trilobites, it may be noted, were
chaste and sober specimens of their race, like the last Roman
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.
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