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.