The Yithians, more formally known "The Great Race of Yith", are a species of body-jumping aliens from the Cthulhu Mythos, first introduced in the story The Shadow out of Time. The title Great refers to the fact they are the only species to have mastered a form of time travel.
They dwelt (or will dwell) on the planet Yith, which may be their homeworld.
The original bodies of the Yithians are never described, since they use their advanced technology to jump their consciousness through time and space into new bodies. The Cretaceous Yithians, who had escaped their own extinction by projecting themselves to Earth, used the body of a large invertebrate creature, who resembled a rugose cone topped by a pair of clawed appendages, a long trumpet (presumably a feeding organ) and a yellow orb-like sensory organ on a stalk, sliding around on a sluglike mucous foot.
The cone like creatures inhabited by the Yithians are asexual and grow children around the bottom of their bodies. Reproduction is rare as a Yithian typically lives for long periods of time. Young Yithians are brought up by there parents, though later familial relationships usually are not given importance. Yithians do however form bonds with the same generational groups.
The Yithians form a loose nation or league run by a small governing board. These governors are voted in by those who pass educational and psychological tests. The nation seems to be split into at least four distinct divisions each given its own board and control of its resources. These are a partly fascistic socialist system. Resources are rationally distributed. All industry and food production is done mechanically. Instead of meat the Yithians only eat vegetable and synthetic foods.
Because most labor is done by machines citizens have much leisure time. This mostly is used in scholarly and aesthetical pursuits. Technology progressed to a height due to the need of surviving the cataclysmic environs of the Yithian culture on Earth. Crime barely exists and legal procedures are carried out with complete respect to the criminal. Civil wars are common inside the Yithian nation, though it often fights the prehistoric civilizations around it, including the Old Ones. But more impressively a huge army is kept on hand with "camera like weapons which produced tremendous electrical effects." This army was to defend the race from the Flying Polyps that were trapped under the Yithian cities.
The topic of the Flying Polyps is the only taboo in the Yithian culture. The Polyps where originally masters over the conical race that the Yithians came to inhabit. When the Yithians projected their minds to that of the conical creatures the polyps were taken by surprise and the Yithians were able to trap them in their black ruins underground. The Yithians considered these polyps to be so evil that they keep an entire army ready to combat any escaping polyps and eventually flee the minds of the cone like creatures into the future.
The Yithians use their mind-projecting techniques to scout out the passage of time, allowing them to accurately predict their own extinction and avoid it. To do this, the mind of a specimen from a (usually sentient) species is swapped with that of a Yithian for a period of several years, so that the Yithian can record information from that time. The target specimen, if willing to cooperate, is allowed to freedom to roam libraries and communicate with other specimens until his time of return comes. The memories of living in the cone-body, however are suppressed before the specimen's mind can return, to secure the secrets.
When the alien Flying Polyps reemerged to doom the civilization of the Cretaceous Yithians, the race, who had long ago predicted this, mass-moved their species' consciousnesses into the "Coleopterous race", beetle-folk living in Earth's far future. It is beleived that due to this, the Yithians will exist until the very end of the universe.
Timeline of Yithian Mind Transfer
6 million years ago: A member of a race of "insect-philosophers" which hail from the fourth moon of Jupiter exchanges minds with one of the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
850,000 years ago: A king of Lomar is among those that exchange minds with one of the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
c. 50,000 BC: A race of "great-headed brown people" dominate South Africa. One of their generals is among those who exchange minds with one of the Yithians. ("The Shadow Out of Time," Lovecraft)
Between 1640 and 1674 BC: Khephnes, who lives during the Fourteenth Dynasty of Egypt, learns the secrets of Nyarlathotep. He is also among those who temporarily exchanges minds with one of the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
c. 200 BC: Theodotides, a Greco-Bactrian official, is among those that exchanges minds with one of the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
Between 82 BC and 75 BC: During the time of the Roman dictator Sulla, a quæstor named Titus Sempronius Blaesus is among those that exchange minds with one of the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
AD 9: The battle of Teutoberger Wald occurs; among the Germanic tribesmen in the battle was Wolfred Herman Freimann, who later exchanged minds with a Yithian. (The Transition of Titus Crow, Lumley)
Bartolomeo Corsi, a Florentine monk, is possessed by one of the Great Race of Yith, and subsequently goes mad. He is eventually exiled to the island of Stromboli and writes the Harmaticon. ("The Shadow Out of Time," Lovecraft; Encyclopedia Cthulhiana 2nd Ed., Harms)
Between 1610 and 1643: Pierre-Louis Montagny, an aged Frenchman living during the reign of Louis XIII, is among those that exchange minds with a Yithian. ("The Shadow Out of Time," Lovecraft)
Between 1651 and 1658: During Oliver Cromwell's time as Lord Protector of England, James Woodville of Suffolk exchanges minds with a Yithian.
2518: Australian physicist Nevel Kingston-Brown dies. In his life, he was one of those who exchanged minds with a Yithian. ("The Shadow Out of Time," Lovecraft)
c. 5000: The empire of Tsan-Chan comes into being. The scholar Yiang-Li, known for his "overviews" and among those who exchanges minds with a Yithian, lives in this empire. ("The Shadow Out of Time," Lovecraft)
c. 16,000: The wizard Nug-Soth is one of many across time who exchanges minds with one of the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
c. AD 18,000,000: A "half-plastic denizen" of the interior of a planet beyond Pluto is among those that exchanges mind with the Great Race of Yith. ("The Shadow Out of Time," Lovecraft)
c. AD 500,000,000: The first primitive life develops on Venus. One Venusian is among those that exchanges minds with the Great Race of Yith. (The Transition of Titus Crow, Lumley; "The Shadow Out of Time," Lovecraft)