Chapter IX: How the Evolution of Senses expands the World

The whole progression of life upon the globe, in so far as it has been achieved through natural selection, has consisted in the preservation and the propagation of those living creatures in whom the adjustment of inner relations to outer relations is most successful. This is only a more detailed and descriptive way of saying that natural selection is equivalent to survival of the fittest. The shapes of animals, as well as their capacities, have been evolved through almost infinitely slow increments of adjustment upon adjustment. In this way, for instance, has been evolved the vertebrate skeleton, through a process of which Spencer's wonderful analysis is as thrilling as a poem. Or consider the development of the special organs of sense. Among the most startling disclosures of embryology are those which relate to this subject. The most perfect organs of touch are the _vibrissŠ_ whiskers of the cat, which act as long levers in communicating impulses to the nerve-fibres that terminate in clusters about the dermal sacs in which they are inserted. These cat-whiskers are merely specialized forms of such hairs as those which cover the bodies of most mammals, and which remain in evanescent shape upon the human skin imbedded in minute sacs. Now in their origin the eye and ear are identical with _vibrissŠ_. In the early stages of vertebrate life, while the differentiations of dermal tissue went mostly to the production of hairs or feathers or scales, sundry special differentiations went to the production of ears and eyes. Embryology shows that in mammals the bulb of the eye and the auditory chamber are extremely metamorphosed hair-sacs, the crystalline lens is a differentiated hair, and the aqueous and vitreous humours are liquefied dermal tissue! The implication of these wonderful facts is that sight and hearing were slowly differentiated from the sense of touch. One can seem to discern how in the history of the eye there was at first a concentration of pigment grains in a particular dermal sac, making that spot exceptionally sensitive to light; then came by slow degrees the heightened translucence, the convexity of surface, the refracting humours, and the multiplication of nerve-vesicles arranging themselves as retinal rods. And what was the result of all this for the creature in whom organs of vision were thus developed? There was an immense extension of the range, complexity, and definiteness of the adjustment of inner relations to outer relations; in other words, there was an immense increase of life. There came into existence, moreover, for those with eyes to see it, a mighty visible world that for sightless creatures had been virtually non-existent.

With the further progress of organic life, the high development of the senses was attended or followed by increase of brain development and the correlative intelligence, immeasurably enlarging the scope of the correspondences between the living creature and the outer world. In the case of Man, the adjustments by which we meet the exigencies of life from day to day are largely psychical, achieved by the aid of ideal representations of environing circumstances. Our actions are guided by our theory of the situation, and it needs no illustration to show us that a true theory is an adjustment of one's ideas to the external facts, and that such adjustments are helps to successful living. The whole worth of education is directed toward cultivating the capacity of framing associations of ideas that conform to objective facts. It is thus that life is guided.