Consider how a complex organ such as the eye evolves over millions of years. Let’s assume some mutations have sensitized an area of the skin to light, so that certain areas are more sensitive to light. What good is this? By the theory, this has no benefit to the species so would be selected out. But let’s suppose it survives, and goes on to develop into an eye, complete with lens, retina, and the muscles needed to focus automatically. How this happens over millions of years is unknown, for unless the entire eye is complete, it is useless. But let’s assume it happened; the eye is complete. Without the nerve connecting it to the brain, and the brain being able to process the information, the partially formed eye is useless, and would be selected out since it gives no advantage to the species. Millions of years of mutations and natural selection wasted.

But let’s suppose all this happened over millions of years. The eye is complete, and the connection to the brain works, and the brain is capable of processing the information. How did two eyes develop? What told the species you needed two? Since you wouldn’t know the benefits of two eyes working together to provide depth perception unless you already had two, how did this develop? If it was random, why don’t we find animals with eyes in the back of their head? Wouldn’t that be useful to see what is behind? The more thinking you do on this, the more absurd it sounds. Darwin himself knew this was a problem, and the passage of time since he proposed his theory has not made it any less of a problem.

To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd to the highest degree. (Charles Darwin The Origin of Species Bantam Books June 1999, page 155)

Darwin explained his problem by making two points he felt would solve this problem:

Numerous examples of species with transitional forms. Unfortunately, this is not the case. As we shall see in another section, the fossil record is silent on transitional forms. Has anything been found with partial eyes?
Each transitional form is useful to the species. This is difficult to imagine. What good is a partially formed eye? A partially formed eye is useless, according to the theory it would be selected out.
The mystery of complex organs poses a huge problem for evolution; partially formed organs are useless. Consider also reproductive organs. If these developed over millions of years the same problem presents itself. What good are partially developed reproductive systems? They serve no purpose so would be selected out. And how did a species reproduce during the millions of years they were developing? If a species could reproduce without them, why would they evolve in the first place?

This is an embarrassing problem