Modern Theosophy has passed through two distinct eras, often mistaken for a single one. The earlier one is represented by the writings of H. P. Blavatsky and her contemporaries; the later, by Charles Webster Leadbeater and Annie Besant. Blavatsky made some remarks on Atlantis and prehistoric cultures in Isis Unveiled (1877), but it was a series of letters signed by the Mahatmas Koot Hoomi and Morya that outlined the definitive scheme of human evolution from the occultist point of view. Blavatsky amplified it in The Secret Doctrine (1888), which chronicles five “root races” that have nothing to do with the colored races of French Atlantology, but are large-scale stages of human evolution. They begin with humanity in an ethereal or gaseous state (the first two root races) and devolve through a gradual coagulation and division into sexes (the third, called Lemurian) into full physicality with the fourth root race (Atlantean). With the fifth (Aryan: our current state), we are on the long return journey to spiritualization.
The rest of his reflections on the subject can be read here.