. Comparative anatomy. Anatomy, Comparative. THE INTEGUMENTARY SYSTEM 175 Although in general the direction of hair growth is such as to make gravity the determining influence, it is a curious fact that the hair on the human forearm suggests his animal ancestry. The hair of the fore- arm slants from the wrist toward the elbow, in the reverse direction to the slant on the upper arm. Man shares this pecuHarity with the apes alone. All other mammals have the same hair direction on both parts of the limb. Why this resemblance of man to the apes unless they share a common ancestry? The pecuHarity i
. Comparative anatomy. Anatomy, Comparative. THE INTEGUMENTARY SYSTEM 175 Although in general the direction of hair growth is such as to make gravity the determining influence, it is a curious fact that the hair on the human forearm suggests his animal ancestry. The hair of the fore- arm slants from the wrist toward the elbow, in the reverse direction to the slant on the upper arm. Man shares this pecuHarity with the apes alone. All other mammals have the same hair direction on both parts of the limb. Why this resemblance of man to the apes unless they share a common ancestry? The pecuHarity is not adaptive, and it is not easy to see why, if man and apes were independently created, they should re- semble one another in this detail. Hair Arrangement. That the arrangement of hairs on the human body has any evolutionary meaning is, to say the least, surprising. Indeed, since such patterns can have no use, we should hardly expect to find them at all. No less surprising is an arrangement of hair in mammals that indicates descent from scaly ancestors. In most mammals, the hairs occur in groups of three or more. These groups are arranged in parallel rows in such wise ^^^ , of the that each cluster lies opposite an interval hairs in groups of threes and fives in the rows in front and behind. In ^^ the human embryo, with the probable ancestral arrangement oi short, the arrangement is imbricated, the scales. (From Kingsiey, after like the universal arrangement of scales. Stohr.) This arrangement, though quite useless, is precisely what we should expect if mammals have descended from scaly ancestors. Histogenesis of Hairs. Hairs are, in origin, epidermal, and therefore ectodermal. Each begins as a minute epidermal papilla, which has arisen by local cell proliferation in the stratum germinativum. Continued proUferation gradually converts this papilla into a cellular column, which extends obhquely downward into the underlying mesenchyma which is to become the cori
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