There is also another factor at work. In our likes and dislikes we do take the condition of hair into account, and in this way, as Darwin pointed out, individuals with hairless bodies, but with rich heads of hair, may have had the preference in sexual choice. We have already alluded to the fact that the sexual glands have a marked influence on the growth of hair which takes place at puberty.
The evolution of hair and of nails hardly comes within the compass of a book dealing only with the human body. The hairy covering of the body became evolved when the mammalian stock was emerging from a very primitive scale-covered ancestry. The hairs were at first produced under the scales, as may still be seen in some low forms of mammals, such as the armadillo and scaly ant-eater. In our bodies it may still be observed that the hair roots are arranged in definite groups of 3, 4 or 5, and it is supposed that such groups represent the hairs which appeared beneath one scale. The evolution of our nails also covers a long period of time. The claw-like nails of the primitive mammals first assumed a flattened shape in the thumb and great toe of the earliest primates. Whenever the digits became truly grasping organs the nails became flattened. In the monkeys of the old and new world the nails are still narrow and much curved from side to side, especially on the little finger and toe. In the great anthropoids they become more human in shape. Long and narrow nails in man, much curved from side to side, are really anthropoid in character. Nails then are not an especial human feature, they were evidently evolved long before the higher primates broke up into the present families.
The little toe especially, and also the corresponding finger, often show signs of retrogression. Not only are their segments or phalanges small, but the one on which the nail is set may fail to be separated from the middle phalanx. The muscles of the fifth digit are often diminished in size and even one or more of them may be reduced to a mere fibrous cord. The nail of the fifth digit often reverts to the claw-like form seen in the very lowest primates.
To appreciate certain features of the organ of taste, it is necessary to mention the arrangement of parts within the mouth and throat. We are accustomed to think of the tongue as a flat movable structure in the floor of the mouth. When, however, special means are taken to study the tongue in the living it is seen to be made up of two surfaces or parts, one situated in the floor of the mouth, and a posterior or deep part which looks backwards into the throat or pharynx. The part of the tongue within the mouth is covered with rough papillae, and is used to move the food during mastication. As long as the food rests on this part of the tongue we still retain control over it; when, however, it passes backwards into the throat and touches the pharyngeal part of the tongue, it passes beyond our control and we must perforce swallow it. The front or mouth part of the tongue we may call the voluntary area; the posterior, deep or pharyngeal part the involuntary. Now at the line where these two areas of the tongue meet there is set a row of special papillae. On these papillae certain groups of cells (taste buds) are placed, and are linked up with the ninth cranial or gustatory nerve. When food comes in contact with these papillae we experience the full sensation of taste. Before it can really come in complete contact with the papillae the food has to cross the border between the voluntary and involuntary parts of the tongue where the taste papillae are set. You see how cunningly nature can set a trap; she neither permits you to waste food nor to neglect the needs of the body; you have to swallow food and thus supply the needs of the body before the taste can be really enjoyed. In this respect we are in the same position as other mammals.
The manner in which the olfactory sense organ is developed has been already alluded to. Mention was also made on a former page (26) of the fact that the olfactory nerves were the only ones connected with the cerebral hemispheres, and that there is every reason for supposing that those great masses of nerve tissue which constitute the main mass of the human brain originally arose in connexion with the sense of smell. In mammals generally every one has observed that the nose is the dominant sense organ. It is plain that a dog's highest mental joys are smell memories. In the higher primates, especially in the civilized races of mankind, the sense of smell has fallen into a minor place among our special senses. We have no words to express our sensations of smell, and yet many of our most realistic memories of childhood are often associated with some impression which reached us first through the nose. The olfactory sense cells are situated only in the roof and upper part of the nasal cavities ; we must sniff in order that the air inspired may come in contact with the sensory membrane.
In the human nose there still remains a trace of a peculiar outpost of the organ of smell. In herbivorous animals, like the sheep or ox, it is manifestly advantageous that the odour of each mouthful of grass should pass into the nose so as to give warning when a noxious plant has entered. Two openings on the roof of the mouth—the naso-palatine— are situated on the palate, just behind the upper gum. Above the openings, on each side of the septum of the nose, is placed a scroll of cartilage containing a detached part of the olfactory mucous membrane—a minute or secondary nose to sample the odours which escape into the nose from each mouthful of food. This little nose is named the organ of Jacobson, after the anatomist who discovered it. These openings on the roof of man's mouth are now closed, but they are clearly represented by vestiges, and so are the o scrolls of cartilage and the detached piece of olfactory membrane. In the anthropoids the organ is also vestigial. How are we to explain these things ?