[9] See Plates VII. and VIII., p. 166, in "Science from an Easy Chair," Second Series, for careful drawings of the complete series of teeth in both the upper and lower jaw of Man and of an Ape.
In the highest apes as well as the lower, the bony lower jaw slopes gradually backwards and downwards from the _palisade_ of front chisel-like teeth or incisors (see Fig. 23 C, p. 277). There is no bony projection below the front teeth--in fact, no bony "chin." But in all modern races of men the front part of the semicircle arch of teeth has shrunk or "withdrawn" considerably or more than has the bony jaw in which the teeth are set. Consequently the bone projects in front of the front teeth as the bony chin (see Fig. 23 A, p. 277, and also "Science from an Easy Chair," 1910, pp. 404, 405). This is characteristic of modern races of man and occurs in no other animal. The very remarkable fact has recently been established that in the ancient species of man from the Middle and Lower Pleistocene--the Neander man and the Heidelberg man (Homo Neanderthalensis)--this extra or excessive shrinking of the dental arch (the half-circle formed by the complete row of teeth) had not taken place. Though the teeth are placed closely side by side and have the same shape as in modern man, they are a little bigger and form a larger and longer arch--more like a horse-shoe than a semicircle, and have not shrunk back so as to leave a projecting bony chin. The bony jaw _recedes_ in these early races of men from the line of the front teeth as it does in the apes. _They have no chin_ (see Fig. 25, p. 286).
Since we are all accustomed to regard a well-marked chin as a necessary feature of a beautiful human face, and to deplore or disapprove the receding or evanescent chin, it is not improbable that sexual selection has favoured the recession of the dental arch with the retention of the original bulk of the lower front margin of the jaw and chin, though why the chin should be thus appreciated is a matter of speculation. It is remarkable that in many of the monkeys the hair grows forward as a projecting beard on the front of the jaw, so as to resemble a chin although no chin is there. It is also the fact that some uncivilized races of men trim the beard and train it in a forward growth so as to suggest the possession of a very prominent chin, when in reality their solid chins of flesh and bone are not especially large.