The work opens with an extensive examination of domestic dogs and cats, exploring whether distinct breeds stem from single or multiple wild species by analyzing osteological data, history, and the recurrence of wild traits such as leg and spinal stripes in duns. The author then turns to horses and asses, detailing the diversity of breeds, the permanence of pedigree, and the curious persistence of ancestral striping as a form of reversion. In the subsequent section, pigs, cattle, sheep, and goats are compared across domestic and feral states, noting how rich food and altered habits modify skulls, tusks, and general proportions, while crossing different lines maintains full fertility. A dedicated section on domestic rabbits investigates the skeletal consequences of confinement and disuse, showing how shortened limbs, reduced brain capacity, and altered skulls parallel the variation found in wild hares.
Finally, the extensive study of domestic pigeons traces numerous specialized races—from pouters and carriers to fantails and tumblers—back to a single ancestral rock-pigeon, illustrating the remarkable power of human selection over generations. Throughout, the author argues that organic beings possess an inherent tendency to vary when exposed to new conditions of life. While man cannot create variability, he accumulates slight inherited differences through methodical or unconscious selection, gradually adapting breeds to his own uses. The text establishes that domestic races often exhibit abnormal characteristics compared to wild species because they have been modified for human benefit rather than their own survival in a state of nature.
