The Universal Kinship was endorsed by a number of contemporary figures including Henry S. Salt, Mark Twain and Jack London, Eugene V. Debs and Mona Caird.

Moore was also connected to the broader humanitarian movement of the late 19th and early 20th centuries, which aimed to improve the treatment of both animals and humans.

To support his claims, Moore drew "extensively upon the fields of geology, paleontology, and biology, together with the works of evolutionary scientists such as Darwin, Huxley, Haeckel, Romanes, and Lubbock.

Moore then delves into the classification of humans as vertebrates and mammals, pointing out shared characteristics such as internal skeletons, four-chambered hearts, and the ability to nourish offspring with milk.

Moore emphasizes the close relationship between humans and other primates, particularly the anthropoid apes, noting similarities in anatomy, behavior, and evolutionary history.

He begins by exploring the conflict between science and tradition, highlighting how religious and cultural beliefs have long promoted the idea of humans possessing a unique, divinely granted soul or consciousness.

However, scientific advancements in biology, psychology, and ethology have increasingly shown that many animals exhibit behaviors and cognitive processes similar to those of humans.

In providing evidence of psychical evolution, Moore argues that mental faculties have evolved in animals just as physical traits have, with different species demonstrating abilities like problem-solving, social structures, and communication skills.

However, Moore critiques the persistence of anthropocentric ethics, which place human interests above all other forms of life, often justifying the exploitation and mistreatment of animals.

In advocating for the doctrine of Universal Kinship, Moore calls for a reevaluation of our ethical principles, urging humans to acknowledge their moral responsibilities toward other creatures.

By emphasizing our shared kinship with all life forms, Moore argues for an ethical framework that recognizes the inherent worth of every living being.

The English writer Henry S. Salt, Moore's friend and fellow animal rights advocate, later described the book, in his autobiography, as "the best ever written in the humanitarian cause".

"[9] English feminist and writer Mona Caird, was so deeply moved by the book that she wrote Moore a personal letter, declaring:[10] It leaves me in a glow of enthusiasm and hope.

That which we have been thinking and feeling—some in one direction and some in another, some in fuller understanding and breadth, others in little flashes of insight here and there—all seems gathered together, expressed, and given form and color and life in your wonderful book.

"[13] J. R. Stanton in American Anthropologist was also critical, stating "[i]ts failing, as in the case of so many works of similar nature, is that in sweeping away impassable gulfs it ignores real differences.