Dentition.--In the natural history of mammals the teeth play an important part as a means of determining differences and relationships. "Everyone knows the milk teeth and the permanent teeth of man. The anthropoid apes bear in this respect an astonishing likeness to man. The number (thirty-two in the adult), the form and general arrangement of the crown, are identical in man and anthropoid apes. The differences are to be found only in minor details." [159] "But the fact must not be lost sight of that all these differences are less pronounced than those which exist between the dentition of anthropoid apes and that of all other monkeys." [160] The Foot.--Anti-evolutionists have laid great stress on the difference between the foot of a man and that of an anthropoid ape. But it is clearly shown by Huxley that in all essential respects the hinder limb of the gorilla terminates in as true a foot as that of man, [161] and "that, be the differences between the hand and foot of man and those of the gorilla what they may, the differences between those of the gorilla and those of the lower apes are much greater." [162] The Sacrum.--"In monkeys, as a whole, the sacrum is composed of three, or rarely four, vertebræ, while in anthropoid apes it contains five--that is to say, just as many as in man." [163] The Skull.--Here the differences are more marked; but again we must remind ourselves that, as regards the osteology, Professor Huxley tells us that "for the skull, no less than for the skeleton in general, the proposition holds good that the differences between man and the gorilla are of smaller value than those between the gorilla and some other apes." [164] The Brain.--Several distinguished zoologists at one time insisted on the absence in all monkeys of certain parts of the brain peculiarly characteristic of man, but now it is unanimously accepted that the parts of the brain in question are "precisely those structures which are the most marked cerebral characters common to man with the apes. They are among the most distinctly simian peculiarities which the human organism exhibits." [165] The difference between the brain of the orang and that of man is a mere difference of degree, and not of kind; and most students of comparative psychology now admit that the intellectual faculties of animals differ from those in man in degree only, not in their essence. Replying to his opponents, Professor Huxley compares the brain of man and that of ape with two watches, one of which will, and the other will not, keep accurate time. He exclaims: "A hair in the balance-wheel, a little rust on a pinion, a bend in a tooth of the escapement, a something so slight that only the practised eye of the watchmaker can discover it, may be the source of all the difference." [166]