It has because of necessity and evolutionary time. This is not evidence for a creator. A category is chosen e.g. the size of a hole and objects are sorted according to this, e.g. stones are small and fit through the hole or are too big and don't. These small stones can then be seived again so that a different category os chosen. Evolution is like this, cummulative selection not single step selection.
Cummulative selection takes the products of the last selection and goes on.
Dawkins illustrates that everything is possible in time. Darwinian selction is cummulative and therefore not random as choices are made according to the environment. Dawkins then talks about a computer programme which he created which writes a programme simulating splitting cells. But the smaller the jump, the less likely death is, and the more likely it is that the jump will result in improvement. This leads on to Chapter 4 which describes examples of single step evolution.