Why is this? Advantage might occasionally be derived if the back of the hand could tell us more than it does about the shapes of the surfaces touched. Why should the thigh near the knee be twice as perceptive as the middle of the thigh? And, last of all, why should the middle of the forearm, middle of the thigh, middle of the back of the neck, and middle of the back, all stand on the lowest level, as having but one thirtieth of the perceptive power which the tip of the forefinger has? To prove that these differences have arisen by natural selection, it has to be shown that such small variation in one of the parts as might occur in a generation—say one tenth extra amount—has yielded an appreciably greater power of self-preservation, and that those inheriting it have continued to be so far advantaged as to multiply more than those who, in other respects equal, were less endowed with this trait. Does any one think he can show this?
But if this distribution of tactual perceptiveness can not be explained by survival of the fittest, how can it be explained? The reply is that, if there has been in operation a cause which it is now the fashion among biologists to ignore or deny, these various differences are at once accounted for. This cause is the inheritance of acquired characters. As a preliminary to setting forth the argument showing this, I have made some experiments.
It is a current belief that the fingers of the blind, more practiced in tactual exploration than the fingers of those who can see, acquire greater discriminativeness: especially the fingers of those blind who have been taught to read from raised letters. Not wishing to trust to this current belief, I recently tested two youths, one of fifteen and the other younger, at the School for the Blind in Upper Avenue Road, and found the belief to be correct. Instead of being unable to distinguish between points of the compasses until they were opened to one twelfth of an inch apart, I found that both of them could distinguish between points when only one fourteenth of an inch apart. They had thick and coarse skins; and doubtless, had this intervening obstacle so produced been less, the discriminative power would have been greater. It afterward occurred to me that a better test would be furnished by those whose finger-ends are exercised in tactual perceptions, not occasionally, as by the blind in reading, but all day long in pursuit of their occupations. The facts answered expectation. Two skilled compositors, on whom I experimented, were both able to distinguish between points when they were only one seventeenth of an inch apart. Thus we have clear proof that constant exercise of the tactual nervous structures leads to further development.
- Let me here note in passing a highly significant implication. The development of nervous structures which in such cases takes place, can not be limited to the finger-ends.