The clearest proofs that Spencer finds of the inheritance of acquired characters are in the well-known experiments of Brown-Séquard. These experiments will be more fully discussed below. Amongst the other morbid processes that Spencer thinks furnish evidence in favor of this view, are cases of a tendency to gout, the occurrence of mental tricks, musical prodigies, liability to consumption, in all of which cases the fundamental distinction between the inheritance of an acquired character and the inherited tendency toward a particular malady is totally ignored.

Twenty-seven years later (in 1893) Spencer took up the open challenge of the anti-Lamarckian writers, and by bringing forward a number of new arguments attempted to reinstate the principle of the inheritance of acquired characters. His first illustration is drawn from the distribution of the sense of touch in different parts of our bodies. Weber’s experiments have shown that if the sharp points of a pair of compasses are applied to the tips of the forefingers, the sensation of two separate points is given when the points are only one-twelfth of an inch apart, and if the points are moved nearer together, they give the sensation of only one point. The inner surfaces of the second joints of the fingers can only distinguish two points when they are one-sixth of an inch apart. The innermost joints are less discriminating, and are about equal in the power of discrimination to the tip of the nose. The end of the big toe, the palm of the hand, and the cheek discriminate only about one-fifth as well as do the tips of the fingers. The back of the hand and the top of the head distinguish only about one-fifteenth as well as the finger-tips. The front of the thigh, near the knee, is somewhat less sensitive than the back of the hand. On the breast the points of the compasses must be separated by more than an inch and a half in order to give two sensations. In the middle of the back the points must be separated by two and a half inches, or more, in order to give two separate impressions.

What is the meaning of these differences, Spencer asks. If natural selection has brought about the result, then it must be shown that “these degrees of endowment have advantaged the possessor to such an extent that not infrequently life has been directly or indirectly preserved by it.” He asks if this, or anything approaching this, result could have occurred.

That the superior perceptiveness of the forefinger-tip might have arisen through selection is admitted by Spencer, but how could this have been the case, he asks, for the middle of the back, and for the face? The tip of the nose has three times more power of discrimination than the lower part of the forehead. Why should the front of the thigh near the knee be twice as perceptive as in the middle of the thigh; and why should the middle of the back and of the neck and the middle of the forearm and of the thigh stand at such low levels? Is it possible, Spencer asks again, that natural selection has determined these relations, and if not, how can they be explained? His reply is that the differences can all be accounted for on the theory of the inheritance of use, for it is evident that “these gradations in tactile perceptiveness correspond with the gradations in the tactual exercise of the parts.” Except from contact with the clothing the body receives hardly any touch sensations from outside, and this accounts for its small power of discrimination. The greater sensitiveness of the chest and abdomen, as compared with the back, is due to these regions being more frequently touched by the hands, and is also owing to inheritance from more remote ancestors, in which the lower surface of the body was more likely to have come in contact with foreign objects than was the back. The middle of the forearm and of the thigh are also less exposed than the knee and the hand, and have correspondingly the power of tactile discrimination less well developed.

Weber showed that the tip of the tongue is more sensitive than any other part of the body, for it can distinguish between two points only one twenty-fourth of an inch apart. Obviously, Spencer says, natural selection cannot account for such extreme delicacy of touch, because, even if it were useful for the tongue to distinguish objects by touch, this power could never be of vital importance to the animal. It cannot even be supposed that such delicacy is necessary for the power of speech.

The sensitiveness of the tongue can be accounted for, however, Spencer claims, as the result of the constant use of the tongue in exploring the cavity of the mouth. It is continually moving about, and touching now one part, and now another, of the mouth cavity. “No advantage is gained. It is simply that the tongue’s position renders perpetual exploration almost inevitable.” No other explanation of the facts seemed possible to Spencer.

Two questions will at once suggest themselves. First, can it be shown that the sensitiveness to touch in various parts of the body is the result of individual experience? Have we learned to discriminate in those parts of the body that are most often brought into contact with surrounding objects? Even the power of discrimination in the tips of the fingers can be improved, as Spencer himself has shown, in the case of the blind, and of skilled compositors. Can we account in this way for the power of discrimination in various parts of the body? In other words, if, beginning in infancy, the middle of the back constantly came into contact with surrounding objects, would this region become as sensitive as the tips of the fingers? The experiment has not, of course, been carried out, but it is not probable that it would succeed. I venture this opinion on the ground of the relative number of the nerves and of the organs of touch on the back, as compared with those of the finger-tips. But, it will be asked, will not the number of the sense-organs become greater if a part is continually used by the individual? It is improbable that much improvement could be brought about in this way. The improvement that takes place through experience is probably not so much the result of the development of more sense-organs, as of better discrimination in the sensation, because the increased power can be very quickly acquired.

An examination of the relative abundance of touch-spots in the skin shows that they are much more numerous in regions of greater sensitiveness. The following table, taken from Sherrington’s account of sense-organs in Schaefer’s “Textbook of Physiology,” gives the smallest distance that two points, simultaneously applied, can be recognized as such (and not simply as one impression) in different regions.

Mm.
Tip of tongue1.1
Volar surface of ungual phalanx of finger2.3
Red surface of lip4.5
Volar face of second phalanx4.5
Dorsal face of third phalanx6.8
Side of tongue9.0
Third line of tongue, 27 mm. from tip9.0
Plantar face of ungual phalanx of first toe11.3
Palm11.3
Back of second phalanx of finger11.3
Forehead22.6
Back of ankle22.6
Back of hand31.6
Forearm, leg40.6
Dorsum of foot40.6
Outer sternum45.1
Back of neck54.1
Middle of back67.1
Upper arm, thigh67.1

The great difference in the sensitiveness of the skin in the different regions is very striking, and if, as seems probable, about the same proportionate difference is found at birth, then the degree of sensibility of the different regions is inborn, and is not the result of each individual experience. Until it can be shown that more of the sense-organs develop in any special part, as the result of the increased use of the part, we have no real basis on which to establish, even as probable, the Lamarckian view.