Such were the beings that invaded the earth when the Second Men were gathering their strength for a great venture in artificial evolution. The motives of the invasion were both economic and religious. The Martians sought water and vegetable matter; but they came also in a crusading spirit, to ‘liberate’ the terrestrial diamonds.

Conditions on the earth were very unfavourable to the invaders. Excessive gravitation troubled them less than might have been expected. Only in their most concentrated form did they find it oppressive. More harmful was the density of the terrestrial atmosphere, which constricted the tenuous animate cloudlets very painfully, hindering their vital processes, and deadening all their movements. In their native atmosphere they swam hither and thither with ease and considerable speed; but the treacly air of the earth hampered them as a bird’s wings are hampered under water. Moreover, owing to their extreme buoyancy as individual cloudlets, they were scarcely able to dive down so far as the mountain-tops. Excessive oxygen was also a source of distress; it tended to put them into a violent fever, which they had only been able to guard against very imperfectly. Even more damaging was the excessive moisture of the atmosphere, both through its solvent effect upon certain factors in the subvital units, and because heavy rain interfered with the physiological processes of the cloudlets and washed many of their materials to the ground.

The invaders had also to cope with the tissue of ‘radio’ messages that constantly enveloped the planet, and tended to interfere with their own organic systems of radiation. They were prepared for this to some extent; but ‘beam wireless’ at close range surprised, bewildered, tortured, and finally routed them; so that they fled back to Mars, leaving many of their number disintegrated in the terrestrial air.

But the pioneering army (or individual, for throughout the adventure it maintained unity of consciousness) had much to report at home. As was expected, there was rich vegetation, and water was even too abundant. There were solid animals, of the type of the prehistoric Martian fauna, but mostly two-legged and erect. Experiment had shown that these creatures died when they were pulled to pieces, and that though the sun’s rays affected them by setting up chemical action in their visual organs, they had no really direct sensitivity to radiation. Obviously, therefore, they must be unconscious. On the other hand, the terrestrial atmosphere was permanently alive with radiation of a violent and incoherent type. It was still uncertain whether these crude ethereal agitations were natural phenomena, mere careless offshoots of the cosmic mind, or whether they were emitted by a terrestrial organism. There was reason to suppose this last to be the case, and that the solid organisms were used by some hidden terrestrial intelligence as instruments; for there were buildings, and many of the bipeds were found within the buildings. Moreover, the sudden violent concentration of beam radiation upon the Martian cloud suggested purposeful and hostile behaviour. Punitive action had therefore been taken, and many buildings and bipeds had been destroyed. The physical basis of such a terrestrial intelligence was still to be discovered. It was certainly not in the terrestrial clouds, for these had turned out to be insensitive to radiation. Anyhow, it was obviously an intelligence of very low order, for its radiation was scarcely at all systematic, and was indeed excessively crude. One or two unfortunate diamonds had been found in a building. There was no sign that they were properly venerated.

The Terrestrials, on their side, were left in complete bewilderment by the extraordinary events of that day. Some had jokingly suggested that since the strange substance had behaved in a manner obviously vindictive, it must have been alive and conscious; but no one took the suggestion seriously. Clearly, however, the thing had been dissipated by beam radiation. That at least was an important piece of practical knowledge. But theoretical knowledge about the real nature of the clouds, and their place in the order of the universe, was for the present wholly lacking. To a race of strong cognitive interest and splendid scientific achievement, this ignorance was violently disturbing. It seemed to shake the foundations of the great structure of knowledge. Many frankly hoped, in spite of the loss of life in the first invasion, that there would soon be another opportunity for studying these amazing objects, which were not quite gaseous and not quite solid, not (apparently) organic, yet capable of behaving in a manner suggestive of life. An opportunity was soon afforded.

Some years after the first invasion, the Martians appeared again, and in far greater force. This time, moreover, they were almost immune from man’s offensive radiation. Operating simultaneously from all the alpine regions of the earth, they began to dry up the great rivers at their sources; and, venturing further afield, they spread over jungle and agricultural land, and stripped off every leaf. Valley after valley was devastated as though by endless swarms of locusts, so that in whole countries there was not a green blade left. The booty was carried off to Mars. Myriads of the subvital units, specialized for transport of water and food materials, were loaded each with a few molecules of the treasure, and dispatched to the home planet. The traffic continued indefinitely. Meanwhile the main body of the Martians proceeded to explore and loot. They were irresistible. For the absorption of water and leafage, they spread over the countryside as an impalpable mist which man had no means to dispel. For the destruction of civilization, they became armies of gigantic cloud-jellies, far bigger than the brute which had formed itself during the earlier invasion. Cities were knocked down and flattened, human beings masticated into pulp. Man tried weapon after weapon in vain.

Presently the Martians discovered the sources of terrestrial radiation in the innumerable wireless transmitting stations. Here at last was the physical basis of the terrestrial intelligence! But what a lowly creature! What a caricature of life! Obviously in respect of complexity and delicacy of organization these wretched immobile systems of glass, metal, and vegetable compounds were not to be compared with the Martian cloud. Their only feat seemed to be that they had managed to get control of the unconscious bipeds who tended them.

In the course of their explorations the Martians also discovered a few more diamonds. The second human species had outgrown the barbaric lust for jewellery; but they recognized the beauty of gems and precious metals, and used them as badges of office. Unfortunately, the Martians, in sacking a town, came upon a woman who was wearing a large diamond between her breasts; for she was mayor of the town, and in charge of the evacuation. That the sacred stone should be used thus, apparently for the mere identification of cattle, shocked the invaders even more than the discovery of fragments of diamonds in certain cutting-instruments. The war now began to be waged with all the heroism and brutality of a crusade. Long after a rich booty of water and vegetable matter had been secured, long after the Terrestrials had developed an effective means of attack, and were slaughtering the Martian clouds with high-tension electricity in the form of artificial lightning flashes, the misguided fanatics stayed on to rescue the diamonds and carry them away to the mountain-tops, where, years afterwards, climbers discovered them, arranged along the rock-edges in glittering files, like sea-bird’s eggs. Thither the dying remnant of the Martian host had transported them with its last strength, scorning to save itself before the diamonds were borne into the pure mountain air, to be lodged with dignity. When the Second Men learned of this great hoard of diamonds, they began to be seriously persuaded that they had been dealing, not with a freak of physical nature, nor yet (as some said) with swarms of bacteria, but with organisms of a higher order. For how could the jewels have been singled out, freed from their metallic settings, and so carefully regimented on the rocks, save by conscious purpose? The murderous clouds must have had at least the pilfering mentality of jackdaws, since evidently they had been fascinated by the treasure. But the very action which revealed their consciousness suggested also that they were no more intelligent than the merely instinctive animals. There was no opportunity of correcting this error, since all the clouds had been destroyed.

The struggle had lasted only a few months. Its material effects on Man were serious but not insurmountable. Its immediate psychological effect was invigorating. The Second Men had long been accustomed to a security and prosperity that were almost utopian. Suddenly they were overwhelmed by a calamity which was quite unintelligible in terms of their own systematic knowledge. Their predecessors, in such a situation, would have behaved with their own characteristic vacillation between the human and the subhuman. They would have contracted a fever of romantic loyalty, and have performed many random acts of secretly self-regarding self-sacrifice. They would have sought profit out of the public disaster, and howled at all who were more fortunate than themselves. They would have cursed their gods, and looked for more useful ones. But also, in an incoherent manner, they would sometimes have behaved reasonably, and would even have risen now and again to the standards of the Second Men. Wholly unused to large-scale human bloodshed, these more developed beings suffered an agony of pity for their mangled fellows. But they said nothing about their pity, and scarcely noticed their own generous grief; for they were busy with the work of rescue. Suddenly confronted with the need of extreme loyalty and courage, they exulted in complying, and experienced that added keenness of spirit which comes when danger is well faced. But it did not occur to them that they were bearing themselves heroically; for they thought they were merely behaving reasonably, showing commonsense. And if any one failed in a tight place, they did not call him coward, but gave him a drug to clear his head; or, if that failed, they put him under a doctor. No doubt, among the First Men such a policy would not have been justified, for these bewildered beings had not the clear and commanding vision which kept all sane members of the second species constant in loyalty.

The immediate psychological effect of the disaster was that it afforded this very noble race healthful exercise for its great reserves of loyalty and heroism. Quite apart from this immediate invigoration, however, the first agony, and those many others which were to follow, influenced the Second Men for good and ill in a train of effects which may be called spiritual. They had long known very well that the universe was one in which there could be not only private but also great public tragedies; and their philosophy did not seek to conceal this fact. Private tragedy they were able to face with a bland fortitude, and even an ecstasy of acceptance, such as the earlier species had but rarely attained. Public tragedy, even world-tragedy, they declared should be faced in the same spirit. But to know world-tragedy in the abstract, is very different from the direct acquaintance with it. And now the Second Men, even while they held their attention earnestly fixed upon the practical work of defence, were determined to absorb this tragedy into the very depths of their being, to scrutinize it fearlessly, savour it, digest it, so that its fierce potency should henceforth be added to them. Therefore they did not curse their gods, nor supplicate them. They said to themselves, ‘Thus, and thus, and thus, is the world. Seeing the depth we shall see also the height; and we shall praise both.’

But their schooling was yet scarcely begun. The Martian invaders were all dead, but their subvital units were dispersed over the planet as a virulent ultra-microscopic dust. For, though as members of the living cloud they could enter the human body without doing permanent harm, now that they were freed from their functions within the higher organic system, they became a predatory virus. Breathed into man’s lungs, they soon adapted themselves to the new environment, and threw his tissues into disorder. Each cell that they entered overthrew its own constitution, like a state which the enemy has successfully infected with lethal propaganda through a mere handful of agents. Thus, though man was temporarily victor over the Martian super-individual, his own vital units were poisoned and destroyed by the subvital remains of his dead enemy. A race whose physique had been as Utopian as its body politic, was reduced to timid invalidity. And it was left in possession of a devastated planet. The loss of water proved negligible; but the destruction of vegetation in all the war areas produced for a while a world famine such as the Second Men had never known. And the material fabric of civilization had been so broken that many decades would have to be spent in rebuilding it.

But the physical damage proved far less serious than the physiological. Earnest research discovered, indeed, a means of checking the infection; and, after a few years of rigorous purging the atmosphere and man’s flesh were clean once more. But the generations that had been stricken never recovered; their tissues had been too seriously corroded. Little by little, of course, there arose a fresh population of undamaged men and women. But it was a small population; for the fertility of the stricken had been much reduced. Thus the earth was now occupied by a small number of healthy persons below middle-age and a very large number of ageing invalids. For many years these cripples had contrived to carry on the work of the world in spite of their frailty, but gradually they began to fail both in endurance and competence. For they were rapidly losing their grip on life, and sinking into a long-drawn-out senility, from which the Second Men had never before suffered; and at the same time the young, forced to take up work for which they were not equipped, committed all manner of blunders and crudities of which their elders would never have been guilty. But such was the general standard of mentality in the second human species, that what might have been an occasion for recrimination produced an unparalleled example of human loyalty at its best. The stricken generations decided almost unanimously that whenever an individual was declared by his generation to have outlived his competence, he should commit suicide. The younger generations, partly through affection, partly through dread of their own incompetence, were at first earnestly opposed to this policy. ‘Our elders’, one young man said, ‘may have declined in vigour, but they are still beloved, and still wise. We dare not carry on without them.’ But the elders maintained their point. Many members of the rising generation were no longer juveniles. And, if the body politic was to survive the economic crisis, it must now ruthlessly cut out all its damaged tissues. Accordingly the decision was carried out. One by one, as occasion demanded, the stricken ‘chose the peace of annihilation,’ leaving a scanty, inexperienced, but vigorous, population to rebuild what had been destroyed.

Four centuries passed, and then again the Martian clouds appeared in the sky. Once more devastation and slaughter. Once more a complete failure of the two mentalities to conceive one another. Once more the Martians were destroyed. Once more the pulmonary plague, the slow purging, a crippled population, and generous suicide.

Again, and again they appeared, at irregular intervals for fifty thousand years. On each occasion the Martians came irresistibly fortified against whatever weapon humanity had last used against them. And so, by degrees, men began to recognize that the enemy was no merely instinctive brute, but intelligent. They therefore made attempts to get in touch with these alien minds, and make overtures for a peaceful settlement. But since obviously the negotiations had to be performed by human beings, and since the Martians always regarded human beings as the mere cattle of the terrestrial intelligence, the envoys were always either ignored or destroyed.

During each invasion the Martians contrived to dispatch a considerable bulk of water to Mars. And every time, not satisfied with this material gain, they stayed too long crusading, until man had found a weapon to circumvent their new defences; and then they were routed. After each invasion man’s recovery was slower and less complete, while Mars, in spite of the loss of a large proportion of its population, was in the long run invigorated with the extra water.