In addition to mutation, recombination, selection, and isolation, biologists have discovered a fifth evolutionary process which is tertiary because it depends on combinations of the other four, only one of which, mutation, is primary. This is a heritable change in the time of appearance of different characters in the growth cycle of the individual.
Each organism passes through three principal stages of development. It starts as an embryo, a fertilized egg in the process of cell division which has not yet reached the point where an em-bryologist can tell its species. In man this condition lasts about nine weeks. Then in mammals it becomes a fetus, in birds a chick, and in insects a larva.5 After it has been born, pecked its way out of its shell, or left its cocoon, it starts on the road to adult life in different stages of preparation, depending on the class of animal it belongs to.
Both in fetal and postnatal life, the individual must be adjusted to its environment, or it will perish. Certain traits that are necessary to the fetus and useless to an adult appear in fetal life and then disappear. Other traits appear as they are needed. Incidentally, it is not true that every individual recapitulates the forms of all its ancestors from the beginning of life on earth. We do, however, recapitulate many of the fetal traits of our ancestors, but not all of them, and not all in the original order. Nevertheless, the etus possesses a vast store of transient genetic characteristics that could be used in adult life under different circumstances.
One of the features that all animals inherit is a built-in timing schedule which regulates the order of appearance and the duration of growth of different bodily systems. This schedule can be upset through standard genetic mechanisms, such as mutation and recombination. The survival of fetal traits into adult life occasioned by such a change is called neoteny.
The classic example of neoteny is the life cycle of an amphibian of the salamander group, the axolotl. This animal arrives at sexual maturity during its tadpole stage and never leaves the water to become an air breather like other salamanders, frogs, and toads, but reproduces and dies in its original medium. Other examples are found among certain birds that have lost the power of flight. They retain throughout life the down that covers the chick before it breaks out of its shell. Ostriches, emus, cassowaries, and penguins have all acquired this neotenous change independently.
In man's ancestors neoteny may have been at play before the appearance of Homo erectus. The position of the head on the neck at right angles to the axis of the vertebral column is neotenous; it is found in the fetuses of all the primates and indeed in those of other mammals. In the fetuses of primates in general the thumb is relatively long in proportion to the length of the other fingers. Among many monkeys and all apes the adult animals have short thumbs, which in man remain neotenously long throughout life.
In insects, which are born fully grown and completely adult, all changes in timing have to be neotenous. In mammals, which are small when born and dependent on their mothers for food and protection, the infantile form differs markedly from the adult in many ways. A baby mammal has to grow mightily and in most species rapidly, and in the higher species it has much to learn. As growth is largely controlled by the endocrines, any shift in endocrine balance can cause radical changes in the form and appearance of the adult animal.
In man some races appear infantile in certain respects throughout life, whereas the children of other races look like miniature adults. In some races the color of the hair never changes during an individual's lifetime, except among persons who reach advanced senility. In others the hair may start out blond, become brown at puberty, and turn white by the age of thirty.
The classbook issued to the members of the Harvard class of 1925 at our twenty-fifth anniversary contains two portraits of each man who was still alive in 1950 and who could be reached. One portrait was taken at graduation, the other twenty-five years later. In some individuals almost no change can be detected; others had changed so much that they were unrecognizable. Yet nearly all these men were of the same racial origin. Age changes, then, vary within populations as well as between them. Not one of my classmates, however, looked like a Pygmy or a Bushman.
Races that retain a number of infantile features throughout life are called pedomorphic; those in which mature features appear early are called gerontomorphic, after the Greek words pais, a child, and geron, an old man. Pedomorphism and gerontomorph-ism are most conspicuous in external, visible anatomy, but they can also affect the nervous system, the vocal cords, other covert systems and structures, and behavior. Most fossil men that we know were gerontomorphic, as witness their heavy brow ridges and long faces. Homo sapiens as a whole seems to be relatively pedomorphic, although variable in this respect both racially and individually.