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II. THE HAIR OF MAMMALS
All kinds of mammalian hair have sufficient similarity to justify the belief that they have had a common phylogenetic origin. Whether hairs were originally protective or sensory has frequently been discussed, but there is considerable likelihood that at first they were neither wholly the one nor the other. Among living mammals, the protective function of hair is obvious, and at the same time there can be no doubt that many hairs, particularly the highly specialized tactile ones, play an important rôle in sense perception. The wide gaps that exist, for example, between movable sensory hairs and fine down or coarse spines are bridged by intermediate forms. That one or another of the various types of hair represents the persistence of an unmodified remnant from the covering of an early mammalian prototype, is not supported by adequate evidence. In the light of what knowledge we have,