Vision And Optical Analysers
The optical analyser developed under the influence of sunlight, so it did not need to be so complicated. The eyes of various animals can perceive no more than a three-octave light flux. The range of light perception is thus only one-fifth that of the sound range. Few animals on our planet are indifferent to light. Even the eyeless protozoa can distinguish light from darkness. Sensitivity to light is based on the property of some chemical reactions to be accelerated in the presence of light. Hence, the protoplasm of practically any cell in a multicellular organism can perceive light, and it needs no eyes for this purpose.
The forerunners of the organ of vision were special light-sensitive cells which could react to feebler light than the other cells in the organism. There are some creatures in which these special light-sensitive cells still exist. One that we know well is the earthworm. It has no eyes, but is quite happy with the numerous light-sensitive cells in its skin. With these cells it feels negligible changes in illumination, which man ca’nnot perceive. It was from such light-sensitive cells scattered all over the body that the eye gradually developed in the course of evolution. At first it was just an accumulation of light-sensitive cells in one spot. Such eyes readily distinguish light from darkness, but they cannot tell from where the light is coming.
The eye then evolved in the following way. The light-sensitive cells gradually acquired a transparent cover and screens of pigment cells that did not allow light into the eye from all directions. Then the light-sensitive spots turned into pits, or even sacs, the first eyes worthy of that name. These eyes could catch light coming only from a certain direction and easily established thereby the direction of the incident light rays. From this primitive optical device there remained but one step to the eye of higher animals: the eye only had to acquire refractive and accommodation systems modifying the refractive index and, last but not least, the oculomotor apparatus that made an active search for visual information possible.