Time Tracks (en Inglés) - John Arthur Neal

A time travel saga spanning millennia, involving a war between humans and dolphins over the destruction of the oceans. It also delves into possible outcomes of current research in brain implants. "Wireheads"-former prisoners, mental patients, elderly terminal cases, those cognitively challenged, and others deemed "hopeless"-become slaves doing the menial jobs. They’re abused, raped, and worked ’til they drop. Through seven time travel trips, our traveler champions these "retreads" and also changes the course of the cetacean war.

In addition, the traveler sees how friends and family members evolve through the centuries as the theory of reincarnation is explored. Apparently, love relationships and innate talents persist from lifetime to lifetime.

Personal and social issues aside, the nature of time itself is questioned. Can free will offset the momentum of genetic inheritance and past experiences? What about the mathematically feasible concept of "causal loops" shaping history from the future?