North American beaver large, webbed hind feet. The unwebbed front paws are smaller, with claws. The eyes are covered by a nictitating membrane which allows the beaver to see under water. The nostrils and ears are sealed while submerged. A thick layer of fat under its skin insulates the beaver from its coldwater environment. The beaver's fur consists of long, coarse outer hairs and short, fine inner hairs (see Double coat). The fur has a range of colors, but usually is dark brown. Scent glands near the genitals secrete an oily substance known as castoreum, which the beaver uses to waterproof its fur.
Cybernetics: Or Control and Communication in the Animal and the Machine derived from Maltz's work. "Cybernetics" became a surprise bestseller and was widely read beyond the technical audience that Wiener had expected. In response he wrote "The Human Use of Human Beings" in which he further explored the social and psychological implications in a format more suited to the non-technical reader. In 1954, Marie Neurath produced a children's book "Machines which seem to Think" , which introduced the concepts of "Cybernetics", control systems and negative feedback in an accessible format. Cybernetics: Or Control and Communication in the Animal and the Machine Cybernetics: Or Control and Communication in the Animal and the