The Eyes in Living Creatures
Let’s now take a look at another scientific fact: A mixture of sand, sawdust, and iron filings is put into a glass box (Illustration A). A magnet is then placed at a particular direction and distance from the glass box. Now, when the box is lifted at its particular position and shaken and juggled for a few seconds, it can instantly be seen that the iron filings attempt to separate itself from the sand and the sawdust, and then gradually starts to shift towards the direction of the magnet (Illustration B). Confined to the walls of the glass ball, the iron filings will eventually gather at that side of the box facing the magnet, which gives us reason enough to say that the iron filings are the most sensitive to the influence of the magnet, when compared with the sawdust and the sand.
Similarly is the case with our eyes: having been conditioned by light, they have now become tissues that are most sensitive to light. Our eyes have thus been originally developed not for us to see with, but rather as a product of the process of conditioning by light.
The presence of animals that have for millions of years survived living in dark places or caves, e.g. the blind cave fish, cave crickets, blind flatworms, cave beetles, and cave crayfish, could perhaps serve to verify this—all these animals do not have eyes developed on them. Were any of them to develop eyes in the middle of their evolution, neither they nor their offspring would ever be able to make use of this sense of sight of theirs as they have been living in the darkness continuously for millions of years. Their sense of sight can no longer function as normally as those of any other creatures that are used to the light.
In fact, there is a case of blind cave fish; the Mexican blind tetra evolved from populations that wandered into caves and eventually settled there. These fish lost the ability to see and their color because it would save resources and energy that could be better directed towards sustainability and reproduction. Also, to not develop eyes in the first place would also prevent them from becoming accidentally damaged or infected, especially now that they are not being actively used.
It can be obviously be seen from the two examples concerning their adaptation to external condition, internal factors did have a role to play too—just like the iron filings in the glass ball when they respond to the influences of the magnet.