The Great Race of Yith are aliens that are the only ones to master time travel. The Great Race are beings of enormous intellectual and psychic powers that once dwelt on the dying world of Yith. They escaped the destruction of their home planet by transferring their minds to the bodies of a species native to Earth in the far distant past.

In the bodies that the Great Race of Yith inhabited on the Earth, they were tall and cone-shaped, rising to a poin with four strange appendages, all of which can extend and recede at will to any distance up to about ten feet. Two appendages have claws, a third with a red 'trumpet' and the forth has a yellowish globe head with three eyes, flower-like ears on top and tentacles on the underside.

The Yithians have no sexes and reproduce by spores instead, though rarely because of their species' longevity.

The unique ability of this scientifically advanced race was to travel through time by swapping minds with creatures of another era. This allowed them to satisfy their interest in human culture, science, and occult beliefs. Once the Great Race had learned all they could from a captive mind, the occupied being's intellect was swapped back. As an additional precaution, the Yithians supressed all knowledge of themselves. However, scraps of knowledge and experiences will remain in dreams.

The Yithians lived on this planet for 200 millions years or so, in fierce competition with the Flying Polyps, whom they initially subdued and sealed behind trap doors. However, this enemy over time increased in numbers and near the close of the Cretaceous era (about 65 million years ago), rose up and finally destroyed the civilisation of the Race of Yith, forcing the Yithians to flee to the bodies of the 'beetle folk' far in the future.