The plan of the proposed new human being was worked out in great detail before any attempt was made to produce an actual individual. Essentially he was to be a normal human organism, with all the bodily functions of the natural type; but he was to be perfected through and through. Care must be taken to give him the greatest possible bulk of brain compatible with such a general plan, but no more. Very carefully his creators calculated the dimensions and internal proportions which their creature must have. His brain could not be nearly as large as their own, since he would have to carry it about with him, and maintain it with his own physiological machinery. On the other hand, if it was to be at all larger than the natural brain, the rest of the organism must be proportionately sturdy. Like the Second Men, the new species must be titanic. Indeed, it must be such as to dwarf even those natural giants. The body, however, must not be so huge as to be seriously hampered by its own weight, and by the necessity of having bones so massive as to be unmanageable.

In working out the general proportions of the new man, his makers took into account the possibility of devising more efficient bone and muscle. After some centuries of patient experiment they did actually invent a means of inducing in germ cells a tendency toward far stronger bone-tissues and far more powerful muscle. At the same time they devised nerve-tissues more highly specialized for their particular functions. And in the new brain, so minute compared with their own, smallness was to be compensated for by efficiency of design, both in the individual cells and in their organization.

Further, it was found possible to economize somewhat in bulk and vital energy by improvements in the digestive system. Certain new models of micro-organisms were produced, which, living symbiotically in the human gut, should render the whole process of digestion easier, more rapid, and less erratic.

Special attention was given to the system of self-repair in all tissues, especially in those which had hitherto been the earliest to wear out. And at the same time the mechanism regulating growth and general senescence was so designed that the new man should reach maturity at the age of two hundred years, and should remain in full vigour for at least three thousand years, when, with the first serious symptom of decay, his heart should suddenly cease functioning. There had been some dispute whether the new being should be endowed with perennial life, like his makers. But in the end it had been decided that, since he was intended only as a transitional type, it would be safer to allow him only a finite, though a prolonged lifetime. There must be no possibility that he should be tempted to regard himself as life’s final expression.

In sensory equipment, the new man was to have all the advantages of the Second and Third Men, and, in addition a still wider range and finer discrimination in every sense organ. More important was the incorporation of Martian units in the new model of germ cell. As the organism developed, these should propagate themselves and congregate in the cells of the brain, so that every brain area might be sensitive to ethereal vibrations, and the whole might emit a strong system of radiation. But care was taken so that this ‘telepathic’ faculty of the new species should remain subordinate. There must be no danger that the individual should become a mere resonator of the herd.

Long drawn out chemical research enabled the Fourth Men to design also far-reaching improvements in the secretions of the new man, so that he should maintain both a perfect physiological equilibrium, and a well-balanced temperament. For they were determined that though he should experience all the range of emotional life, his passions should not run into disastrous excess; nor should he be prone to some one emotion in season and out of season. It was necessary also to revise in great detail the whole system of natural reflexes, abolishing some, modifying others, and again strengthening others. All the more complex, ‘instinctive’ responses, which had persisted in man since the days of Pithecanthropus Erectus, had also to be meticulously revised, both in respect of the form of the activity and the objects upon which they should be instinctively directed. Anger, fear, curiosity, humour, tenderness, egoism, sexual passion, and sociality must all be possible, but never uncontrollable. In fact, as with the Second Men, but more emphatically, the new type was to have an innate aptitude for, and inclination toward, all those higher activities and objects which, in the First Men, were only achieved after laborious discipline. Thus, while the design included self-regard, it also involved a disposition to prize the self chiefly as a social and intellectual being, rather than as a primeval savage. And while it included strong sociality, the group upon which instinctive interest was to be primarily directed was to be nothing less than the organized community of all minds. And again, while it included vigorous primitive sexuality and parenthood, it provided also those innate ‘sublimations’ which had occurred in the second species; for instance, the native aptitude for altruistic love of individual spirits of every kind, and for art and religion. Only by a miracle of pure intellectual skill could the cold-natured Great Brains, who were themselves doomed never to have actual experience of such activities, contrive, merely by study of the Third Men, to see their importance, and to design an organism splendidly capable of them. It was much as though a blind race, after studying physics, should invent organs of sight.

It was recognized, of course, that in a race in which the average life-span should be counted in thousands of years, procreation must be very rare. Yet it was also recognized that, for full development of mind, not only sexual intercourse but parenthood was necessary in both sexes. This difficulty was overcome partly by designing a very prolonged infancy and childhood; which, necessary in themselves for the proper mental and physical growth of these complicated organisms, provided also a longer exercise of parenthood for the mature. At the same time the actual process of child-birth was designed to be as easy as among the Third Men. And it was expected that with its greatly improved physiological organization the infant would not need that anxious and absorbing care which had so seriously hobbled most mothers among the earlier races.

The mere sketching out of these preliminary specifications of an improved human being involved many centuries of research and calculation which taxed even the ingenuity of the Great Brains. Then followed a lengthy period of tentative experiment in the actual production of such a type. For some thousands of years little was done but to show that many promising lines of attack were after all barren. And several times during this period the whole work was held up by disagreements among the Great Brains themselves as to the policy to be adopted. Once, indeed, they took to violence, one party attacking the other with chemicals, microbes, and armies of human automata.

In short it was only after many failures, and after many barren epochs during which, for a variety of reasons, the enterprise was neglected, that the Fourth Men did at length fashion two individuals almost precisely of the type they had originally designed. These were produced from a single fertilized ovum, in laboratory conditions. Identical twins, but of opposite sexes, they became the Adam and Eve of a new and glorious human species, the Fifth Men.

It may fittingly be said of the Fifth Men that they were the first to attain true human proportions of body and mind. On the average they were more than twice as tall as the First Men, and much taller than the Second Men. Their lower limbs had therefore to be extremely massive compared with the torso which they had to support. Thus, upon the ample pedestal of their feet, they stood like columns of masonry. Yet though their proportions were in a manner elephantine, there was a remarkable precision and even delicacy in the volumes that composed them. Their great arms and shoulders, dwarfed somewhat by their still mightier legs, were instruments not only of power but also of fine adjustment. Their hands also were fashioned both for power and for minute control; for, while the thumb and forefinger constituted a formidable vice, the delicate sixth finger had been induced to divide its tip into two Lilliputian fingers and a corresponding thumb. The contours of the limbs were sharply visible, for the body bore no hair, save for a close, thick skullcap which, in the original stock, was of ruddy brown. The well-marked eyebrows, when drawn down, shaded the sensitive eyes from the sun. Elsewhere there was no need of hair, for the brown skin had been so ingeniously contrived that it maintained an even temperature alike in tropical and sub-Arctic climates, with no aid either from hair or clothes. Compared with the great body, the head was not large, though the brain-capacity was twice that of the Second Men. In the original pair of individuals the immense eyes were of a deep violet, the features strongly moulded and mobile. These facial characters had not been specially designed, for they seemed unimportant to the Fourth Men; but the play of biological forces resulted in a face not unlike that of the Second Men, though with an added and indescribable expression which no human face had hitherto attained.

How from this pair of individuals the new population gradually arose; how at first it was earnestly fostered by its creators; how it subsequently asserted its independence and took control of its own destiny; how the Great Brains failed piteously to understand and sympathize with the mentality of their creatures, and tried to tyrannize over them; how for a while the planet was divided into two mutually intolerant communities, and was at last drenched with man’s blood, until the human automata were exterminated, the Great Brains starved or blown to pieces, and the Fifth Men themselves decimated; how, as a result of these events, a dense fog of barbarism settled once more upon the planet, so that the Fifth Men, like so many other races, had after all to start rebuilding civilization and culture from its very foundations; how all these things befell we must not in detail observe.