Page:George Archdall Reid 1896 The present evolution of man.djvu/105

Rh properly be regarded as developmental, vary little or not at all under stimulation; muscle, as we see, normally attains nearly the limits of its power to vary, as also do healthy bone and cartilage; while other structures, such as skin, adipose tissue, and nervous tissue, do not "normally" approach so nearly the limits of their powers to vary. The skin thickens and hardens greatly in response to the stimulation of intermittent pressure or of moderate irritation, or it may expand immensely as over tumours; adipose tissue has a great power of growth; nervous tissue, judged by the function it performs, whether of sensation, perception, thought, &c., has also the power under continued stimulation of varying very greatly beyond the normal limits.

We may now ask ourselves the question—How much of the tactual discriminativeness of the skin in various parts of the body is due to direct inheritance, and how much is individually acquired under the influence of more or less continued stimulation? The answer, I think, must be, that while some of the discriminativeness is doubtless due to direct inheritance,—for in various parts of the body where it is of particular importance (e.g. hands and feet in man, tongue in some animals) are found special nervous elements which subserve the sense of touch,—yet beyond doubt much of it or most of it is not due to direct inheritance, but to the power of the individual organism to vary in correspondence with the environment; the tactual discriminativeness being most developed in those parts where it is oftenest stimulated. In this way may be explained all or most of the superior discriminativeness of the front of the body and the nose, as compared to the back of the body and the lower part of the forehead respectively.