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Similar to the Octopus and squid eyes, vertebrates also have lens-containing eyes. Unlike squids and octopuses, however, vertebrates have a blind spot, and an light-sensitive retina which adjusts the size of the pupil to allow less or more light into the eye (although squids and octopuses can adjust the depth of their lens in response to certain amounts of light). So just as the nautilus made a trade-off for better vision at the price of a dim image, vertebrates have made a trade-off of having a blind-spot and being able to adjust the amount of light entering the eye.

The Theoretical Development of the Vertebrate Eye

Remember back to the previous page, on how a lens can evolve from a non lens-containing eye. Through a little more trial and error, it seems logical that a primitive terrestrial vertebrate would eventually evolve the muscle structure to control the amount of light entering the eye. It would be a major evolutionary advantage to its possessor. It would allow the organism to hunt and/or avoid predators in both highly lit, and dim environments. Just as in all other examples, the fitness of the possessor and all of its kin should have a higher survival rate than those whom do not possess the advantageous trait. Therefore, a new successor shall soon take the place of its brethren in this fiercely competing world.

Disadvantages of the Vertebrate Eye

So it is advantageous to have such an eye as the vertebrate eye. What are the disadvantages of having it?

First, the vertebrate eye, as mentioned before, possesses a blind spot. This is due to the eyes being an extension of the brain, which, upon formation are pinched off and the optic nerve and blood vessels are limited to go through one small hole at the back of the eye (Dawkins, 29). This part of the eye has no photoreceptive cells. This is the reason we have a blind spot. Now this may not seem like a big deal to us, because we have learned how to deal with such an imperfection. But it reminds us that we are definitely not perfect beings.

Second, the vertebrate eye has a thin protective layer, called the cornea, which is easily damaged. This is a big problem, because a scratch in the cornea can render the eye with damaged vision, or worse, it can make the eye useless. Perhaps that is part of the reason to why eyelids evolved in terrestrial vertebrates, along with the thin layer of mucous produced on the surface of the eye.


No matter how it is viewed, the development of the eye is intriguing to say the least. Both in the past and present, the evolution of the eye can be seen through gradual change from one species to the next. It leaves one question remaining: Was the path taken by insects, vertebrates, and the other organisms alive today the only paths for which eyes could form, or were there more types of eyes in our past than we could ever imagine?