of Neptune, a rabbit-like species evolves into the Tenth Men. The Tenth Men are sapient but primitive. After a plague wipes out the Tenth Men, several other primitive human species rise and fall. Fourteenth to Seventeenth Men. The Fourteenth, Fifteenth, and Sixteenth Men are essentially Neptunian versions of the First, Second and Fifth Men, respectively. The Fourteenth Men create great civilisations and destroy them, frustrated with their own imperfections. The Fifteenth improve upon that, creating a spirituality based on "a devotion to the fulfilment of human capacity". They eventually create the Sixteenth Men - the first Neptunian artificial species. Thus the cycle of rise and collapse of civilisations ends, and steady progress takes its place. The Sixteenth Men achieve the highest level of civilisation possible "to the individual human brain acting in physical isolation" and to avoid stagnation create the Seventeenth Men, with an ability for "mental fusion of many individuals" to succeed them; however, the Seventeenth Men are "flawed" in some unspecified way, unimagined by the 16th due to their lesser awareness, and last only a short period of time before being replaced by the Eighteenth Men. Eighteenth Men. (Chapters 15–16) The most advanced humans of all, essentially a perfected version of the 17th species. A race of philosophers and artists with a very liberal sexual morality. "Superficially we seem to be not one species but many." (One interesting aspect of the Eighteenth Men is that they have a number of different "sub-genders," variants on the basic male and female pattern, with distinctive temperaments. The Eighteenth Men's equivalent of the family unit includes one of each of these sub-genders and is the basis of their society. The units have the ability to act as a group mind, which eventually leads to the establishment of a single group mind uniting the entire species.) This species no longer died naturally, but only by accident, suicide or being killed. Despite their hyper-advanced civilisation, they practice ritual cannibalism. They are eventually extinguished on Neptune after a supernova infects the sun, causing it to grow so hot that it consumes the remains of the Solar System, faster than any means of escape they can devise. Unable to escape, this last species of man devises a virus to spread life to other worlds and cause the evolution of new sentient species throughout the galaxy. Sub-humans Baboon-like Submen. (Chapter 7) "Bent so that as often as