This article treats late ancient rabbinic texts (ca. 1st-early 3rd cents. CE), reading them as biology, and following their ideas about the limits and possibilities of reproductive and species variation. I read sources from the tractates of Niddah, Kil’ayim, and Bekhorot, in the Mishnah and Toseta, as expressions of a science of generation, or a biology, in which nonhuman zoology and human gynecology were entwined. I argue that the rabbis, like other ancient thinkers, understood that creatures of a particular kind (or species), including the human-kind, might deliver a creature that appears to be of a different kind. I show that in the majority of cases the Tannaim believed these species nonconforming offspring not to be genuine hybrids, that is, they did not believe that they were the results of cross-species mating. Rather, they understood most species-variation to be spontaneously arising. By reading Bekhorot and Kilayim together, I note how the human features in such cases of species variation, as well as in rabbinic zoological distinctions between *different* kinds that nonetheless looked alike. In other words, I tackle the rabbinic principle that all animals have doubles in the wild and in the seas, including the human.