Biology Reference
In-Depth Information
Chapter 5
The Evolution of Eyes
One of Darwin's Difficulties
One of the many impressive aspects of On the Origin of Species is that Darwin
devotes an entire chapter to what he terms “Difficulties of the Theory”. This is not
a habit common among leading scientists today! One of the difficulties he discusses
is the evolution of eyes. He starts by pointing out what many people feel when
considering this organ in terms of evolutionary theory:
To suppose that the eye with all its inimitable contrivances for adjusting the focus to differ-
ent 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 in the highest degree.
The human eye is superbly adapted to its function of providing detailed visual
information about the world. Figure 5.1 shows a section through the human eye, to
remind you of its basic features.
The eye is roughly spherical, located in a protective bony socket and surrounded
by six muscles. Unlike a camera, the eye forms a sharp image of only a small part
of the visual field at any one time, so these muscles are in constant use to allow
the eye to scan a wide angle, so that the brain can build up a detailed image of the
whole scene. Light enters through a transparent protective cover called the cornea,
whose surface is kept clean by means of secretions from tear glands located above
and at the inner side of the eye. These secretions contain antibacterial proteins. The
light then passes through an opening in the iris called the pupil; the diameter of
this opening is changed automatically to accommodate different light intensities
by means of muscle fibres located in the iris. The iris is coloured to prevent light
entering except through the pupil. Light continues through the lens and is focussed
onto the light-sensitive layer called the retina that lines the inside of the eye. The
lens is made of transparent cells arranged in a biconvex shape. This shape can be
changed to focus on objects at different distances by means of muscles that surround
the lens.
The retina consist of an outer pigmented layer that functions to prevent light
reflecting inside the eye, and an inner layer of nerve cells, some of which are sen-
sitive to light (see Figure 5.1, right-hand diagram). It is this pigmented layer that
Search WWH ::




Custom Search