Of the six principal sense organs three are the most important to us. We can lose the sense of taste and, certainly, smell, and not even bother about it. One could even somehow put up with the loss of the sense of touch, but the loss of vision, hearing, or sense of equilibrium seriously incapacitates a person. These are the principal senses we use to perceive the world we live in.
Man’s main sense organs do not always coincide in importance with the main analysers in animals. Many representatives of the animal kingdom have very feeble vision and some are completely blind. Others are deaf, or only hear very badly, and yet they still manage quite well. As to the organ of equilibrium, this is an extremely important analyser and almost all multicellular animals have it.
Even in unicellular animals zoologists have discovered certain structures remotely resembling the organ of equilibrium of higher animals. The parasitic infusoria possess a device of this sort. They have a special vacuole, a small peripheral sac with some crystalline inclusions, similar in structure to the statocyst (the sac of the labyrinth maintaining static equilibrium) in multicellular animals. If it is some day discovered that the vacuole does indeed fulfill the same function, it will not be surprising. There are many remote corners on our planet plunged in pitch-darkness, many spots where no sound ever penetrates, but the Earth gravity is ubiquitous and inescapable.
There are grounds for believing that light has played an active part in the origination of life. At any rate, light sensitivity, which seems to have been possessed even by primary living matter, soon gave rise to a special organ, the organ of vision. Even contemporary unicellular flagel-lata perceive light. The unicellular animals, particularly the Peridinia, many of which can themselves emit light, have rather large eyes. Their eyes are a bowl-shaped accumulation of reddish, fat-like, light-sensitive pigment located in the anterior part of the Peridinia, at the base of the flagellum. In the pigment there is a transparent grain of starch which serves to refract and focus light.
Of the three main sense organs which are most essential to man, the two oldest are the organs of vision and equilibrium. These organs which are, on the whole, very dissimilar, have one interesting feature in common. Although the organs of vision and equilibrium have greatly modified in the process of their evolution and perfection, they differ less in design and specific function than the acoustic analysers and the sound receptors in various animals. This is obviously because the organs of vision and equilibrium were both shaped under the impact of some single, constant global factor; the sense of equilibrium was formed under the influence of the Earth gravity, and vision under the impact of sunlight. But on the Earth there has never been any unique and standard source of sound.