In Eocene times—70 million years
ago—small primitive mammals rather suddenly gave rise to over
a dozen very different orders: hoofed animals (odd-toed and
even-toed), elephants, carnivores, whales, rodents, bats and
monkeys. Since that time no other orders of mammals have evolved.
There have been great varieties of evolution within the orders that
did appear, but strangely enough major evolution seems to have
halted. Of course there have been many new species formed, but they
all belong to families that appeared millions and millions of years
ago. But a few million years ago a new kind of evolutionary
progress began with the emergence of the hominoids and then of the
man apes, the precursors of the ape men. And with the first true
men—Homo sapiens—evolution begins again, in a new form,
with the evolution of social man that is still going on
today.
The Pleistocene was a period of continuous of
alternating cold and warm phases—climatic factors of some
importance in estimating the migrations of early man and
determining his physical and cultural evolution.
The Pleistocene (or Ice Age) was the last period
of the Cenozoic era, which extends from the end of the Cretaceous
period—the period of the great chalk deposits, some 70
million years ago—to the present time. The Cenozoic era
comprises the following periods: the Eocene, the Oligocene, the
Miocene, the Pliocene and, as we have seen, the Pleistocene.
The principal driving force of genetic evolution
of all organisms studied to date is natural selection, the
differential contribution to the next generation by various genetic
types belonging to the same population. This is the process often
called Darwinism, to distinguish it from mutation and other
conceivable driving forces. The main features of anatomy,
physiology, and behaviour are ultimately ascribable to natural
selection in which changes in environment open and close
opportunities for variations in genotypes to become
established.