The olfactory apparatus in most mammals contains a well-developed portion known as the organ of Jacobson, the probable function of which is to appreciate the flavour of food in the mouth. In man, this organ is in a rudimentary condition and cannot fulfil its function, as it is devoid of its proper nerve. This remnant, now useless, gives us information as to the evolution of the organ of smell in man. In the human fœtus, Jacobson’s organ is not only better developed than in adult man, but it is also provided with a stout nerve trunk, which disappears towards the end of embryonic life. The organ, however, cannot perform any olfactory function. The human fœtus, moreover, possesses five turbinals which later on become reduced to three, and of these only two develop completely.
The history of the evolution of the organ of smell, as it has been made out by comparative anatomy and embryology, links this apparatus in man with the corresponding organs of other mammals by means of these useless rudiments, which, however, are important evidence in scientific theory.
The auditory apparatus also has become reduced in man. Many animals, in the struggle for existence, require a very acute sense of hearing, more so than man or some of the most intelligent mammals. We have all seen how horses raise their ears to hear better when there is the slightest sound near them. Monkeys and man have lost this power, and man sometimes tries to supply the defect by artificial means. When a lecturer, for instance, is not speaking sufficiently loud some of the audience put their hands to their ears, making a kind of trumpet which serves to catch the sound. The human external ear is supplied with muscles, but in most cases these are too feeble to move it. In very rare cases persons can move their ears, the muscles inserted to the shell in most of us being mere rudiments of those that existed in our ancestors.
In the organ of sight, the little fold in the inner angle of the eye, known as the semilunar fold, is of special interest. This membrane is a useless vestige of a structure much better developed in lower mammals. In the dog it is present as a small third eyelid, supported by a special cartilage provided with a secreting gland, known as the Harderian gland. In birds, reptiles and frogs, the corresponding structures are much better developed. Everyone has seen the delicate membrane which, in the case of a bird, may shoot out from the inner angle of the eye and cover the whole of the exposed part of the eyeball (nictitating membrane). In these animals, the eye is protected by this third lid, which has its own muscles. As in the dog, this third eyelid of birds and lower vertebrates is generally provided with a large Harderian gland, which produces a liquid secretion like tears.
In most monkeys, this apparatus is much reduced. Many of them have still a small Harderian gland and a weak third eyelid. In man, as I have already said, there are only vestiges of these organs, the gland being almost atrophied and the third eyelid represented only by an insignificant crescentic fold. In the lower races the fold sometimes contains a small cartilage. Giacomini found it twelve times in sixteen negroes, whilst in 548 white people it was found only in three cases.
The interpretation of these facts is not doubtful. This little fold is the last vestige in use of an organ which was useful only in our remote ancestors.
The organs of reproduction in the human race also show a number of rudiments. There remain even traces of a hermaphrodite condition, a very low degree of organisation, going back to extremely remote ancestors. The evidence given by the very large number of abnormalities that are found in these organs makes it clear that, in the long period of the evolution of the human race, they have been subjected to a series of modifications. Thus, for instance, there is occasionally present in women a form of uterus resembling that of the lower mammals, or even the double uterus of marsupials.
The evolution of man has been dominated by the great development of the brain and of the intelligence, and man, accordingly, has lost many organs and functions which were of use in his more or less remote ancestors.