Understanding eye evolution is easy. Begin with a small change, then add changes, each giving an advantage, until what you have can be called a “new organ”.
First, start with regular temperature sensitive nerve cells (we can evolve their presence in steps too if you like). Now, a mutation causes them to fire when exposed to light instead of heat - not hard since light produces heat anyway. Now there are a clump of light sensitive cells, which are advantageous because sensing light is better than not sensing it - this may allow the little creature to hide under a rock, etc. Now a mutation causes that clump to be depressed in the skin - thus protecting it. Similarly, you can see that the opposite mutation - causing it to bulge up, would be selected against, so the “depressed in” mutation survives. Now the cup with light sensitive cells can sense light direction too! Mutations that cause a transparent cover of cells, and later thicken this layer to make a lens, or add muscles that allow the eye to move are all accumulations of small steps, each being advantageous, up to the eye you possess.
I'm curious about the transition between "c" and "d" on that.
That's understandable. Let's look at it closely. Say a mutation causes a thin layer of clear skin to cover the eye (outer, thin layers of skin are often clear). That has the advantage of protecting the eye, an so seems clear that it would be selected for. That would automatically fill the middle with a fluid due to normal growth (embryos grow in fluid, not in air). Is that what you were unclear on, or is there some other step in the useful image on post #9 that is still a little foggy?
Eye evolution was recognized as easy even 150 years ago - Darwin described the process in post #9, long before we found living transitional forms of so may intermediate steps.
Have a good day-
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