What are the main stages of …

Evolution is a step by step process, it does not happen in few years. It takes generations by generations to exhibit variations over a large range. Organisms get adapted to their habitat by bring about certain changes in their structure. These changes are called as adaptations which bring about variations in the evolution. These variations are carried on to successive generations by natural selection. A classic example which supports the statement 'Evolution by stages' is the evolution of eye.

In lower organisms like planaria, optic sensory structures are present  as patches which act as eye spots.
In organisms like cockroaches, compound eyes are present which help them to see around their body.
In organisms like octopus, eye is remarkable having  iris, circular lens, vitreous cavity, retina with photoreceptor cells. They lack a cornea. The lens in octopus does not change its shape but focusses through movement.
In vertebrates, eye is a well-developed sensory structure with iris, ciliary muscles, pupil, cornea and a lens. The size of the pupil can be regulated. Shape of the lens can be adjusted in order to focus the image on the retina.