The work surveys animal physiology, development, and behavior to infer mental processes in nonhuman animals, opening with organic evolution and the physical bases of life such as respiration and nutrition. It examines reproduction, growth, variation, and heredity, considers mechanisms proposed for inheritance, and explains natural selection, isolation, and adaptation including mimicry and protective resemblance. Attention is given to the contrast between instinct, habit, and learned intelligence, with examples illustrating gradations of mental capacity. Human reasoning and moral agency are treated as distinct yet still subject to underlying biological laws, and scientific evidence is combined with philosophical analysis throughout.