Martian (Last and First Men)

Martians are a sapient species of amorphous creatures which invaded Earth over 10 million years in the future and fought a series of devastating wars against the Second Men for over 350,000 years.

Biology
Martians are cloud-like amorphous creatures which can alter their bodies' density at will. They usually appear like large greenish clouds, but can also contract themselves to a dense aggregation that behaves more like a jelly. Their bodies are made out of minuscule units, much smaller than cells and smaller even than Earth's viruses. Like cells, however, they exist in different forms, specialized to fill different functions within the larger cloud or organism. Their biochemistry is complex, and involves selenium. They gain energy from sunlight, and acquire nutrients by absorbing chemicals from the ground and moisture from the atmosphere, although they can also absorb nutrients from vegetation. Their biological units divide themselves, and the division of several such units can give life to a new individual, which is born as it is ejected from its parental cloud. Since their bodies are constantly renewing themselves, Martians are biologically immortal and can only die from disease or accidents.

Martians communicate by transmitting and picking up electromagnetic radiation in various wave-lengths. The ultramicroscopic units that make up their cloudlet-like bodies generate magnetic fields to repel and attract each other, thus allowing the Martian to alter their body density and shape, move around and manipulate physical objects. By manipulating drops of water of various sizes, Martians can create different kinds of aquous lenses to attach to light-sensitive units and act as eyes of any kind, ranging from microscopic to telescopic vision. At night and during certain parts of the day, they rest upon the ground like a mist and create root-like projections to penetrate the soil and absorb nutrients.

In times of necessity, all Martians can link their minds and operate as a unified planetary consciousness: all individuals thinking and acting as one. Conversely, individual Martians can also "turn off" their consciousness and allow their ultramicroscopic units to operate like independent microbes for a while.

Although they can survive on Earth, Martians struggle to move gracefully in the Earth's dense atmosphere, and their bodies are so light that they tend to float up to the mountaintops. They also find the excessive water and oxygen in the atmosphere extremely uncomfortable, and have difficulty operating in rain. Finally, the presence of radio waves interferes with their own EMR systems, sometimes to the point of being unbearable. Despite such disadvantages, the Martians can be an extraordinarily dangerous enemy pitted against humanoids, who can't harm the Martians directly whereas large concentrations of Martians are perfectly capable of crushing or tearing humanoid bodies apart with brutal effectiveness.

Culture and society
Due to the existence of the group-mind, Martians are a very unified culture. Even when the group-mind is not operative and individuals are still individuals, they still see themselves as part of something greater. They practice agriculture and engage in complex aerial dances which incorporate intertwining with each other and contracting their bodies into solid geometric shapes. Due to their curiosity about the world and being naturally equipped with both telescopic and microscopic vision, Martians have developped scientific knowledge in various fields, which they write down in scrolls of paper kept in libraries of stone.

Martians have a nearly religious admiration for rigid materials, particularly crystals and most especially diamonds, the latter which are the object of actually religious worship. On Mars, all diamonds are venerated, placed on top of sacred buildings where they can be exposed to sunlight. The Martians' admiration for this sacred material is such that they regard the very thought of a diamond not being treated with the proper respect as sacrilegious.

Martians equate EM radiation with intelligence. When invading Earth, they utterly failed to realize that the Second Men (or any other Earth species) could have consciousnesses. At the same time, they believed that Earth's radio-broadcasting stations must be alive and sapient in some way, although they failed to communicate with them.

History
As the resources on Mars became increasingly scarce, and the arid climate increasingly less suitable for agriculture, the Martians were forced to invade Earth, seeking out food and water. Another factor that motivated them was the opportunity to find sacred diamonds on their neighboring planet. Instead of building spaceships, they developed artificial means to keep their bodies alive when traveling through space in their dissipated state, sailing on the gravitational fields of the solar system.

After arriving on Earth, the Martians believed that radio stations were the Earth's native intelligence and that men were merely mindless creatures controlled by the stations. They were shocked to discover diamonds being used for such undignified purposes as to identify "cattle", as they saw it. The invasion was turned into a religious crusade to "liberate" the sacred gems. Despite great losses, the Second Men eventually emerged victorious against the Martian invaders.

Unfortunately, even as the Martians died, some of their bodies' microscopic units survived and became capable of operating independently as common microbes. Inhaled by men, they became pulmonary viruses, and the resulting plague combined with famine and devastation was enough to cripple the vitality of entire generations and weaken the Second Men's civilization for the next four centuries, until the second Martian invasion finally arrived to take more vegetation and water from Earth. This second invasion ended exactly like the first one, but still, for the next 50,000 years the cycle repeated itself. Every few centuries, the Martians raided Earth, carried large quantities of food and water back to their planet, sent waves of religious fanatics to crusade against the "Earth intelligence" (i.e. radio stations), destroy their bipedal servants and "liberate" the diamonds. Every time they were defeated, but new viruses emerged from their bodies to continue to torment the Second Men. Although they always succeeded in defeating the invaders, men's forces were gradually weakened, while Mars benefitted from the loots and came back stronger. After the first 50,000 years had passed, the Martians managed to conquer some areas of Earth, starting with Antarctica and expanding their domains to Australasia and South Africa, where they remained for centuries practicing agriculture and collecting diamonds.

For the next hundreds of millennia, a new and even stranger cycle was established. Every time the Martian colonists on Earth started to develop a different culture and threatened to give rise to a new group-mind to replace or override the one on their homeworld. Each time, therefore, Mars destroyed its own colonists and sent new ones. This cycle repeated itself for 300,000 years, and culminated in a new brutal offensive against the Second Men (who had mostly kept to themselves during that period and resigned to sharing their world with their enemies). This final war was ended by a biological weapon developed by men: a disease that exterminated the Martians on Earth and Mars, but also infected mankind itself, plunging the Earth into its Third Dark Age.

Appearances

 * Last and First Men: A Story of the Near and Far Future, by Olaf Stapledon (1930)
 * Star Maker, by Olaf Stapledon (1937)