Evolution is a beutiful theory. There is overwhelming evidence for it in the form of fossil record, similarities between existing creatures and evident re-use of structural motifs in altered contexts the basic idea of descent, modification and elimination of 'less successful' variants must be in essence correct.
However Darwin had a problem with evolution. He was worried about its ability to perfect structures and its rapidity. As he realised, these two features present very similar problems. The changes, according to Darwin, are almost imperceptible. These near imperceptible change give a survival advantage, however slight to one variant over another and these ultimately result in incredbly well designed structures. Is there enough time for the well designed structures - the classic being the eye - to come about. Darwin felt the answer was yes, but he was uneasy. In 'Origins' there is a chapter on organs of 'extreme perfection' in which he persuasivsly argues that given enough time they could arise through an evolutionary process.
Darwin's concerns about his own theory, however, caused him to revise his opus 'The Origin of Species', he produced six editions in all. We use the second, and the later editions are largely ignored. In later issues Darwin introduced pseudo-Lamarkian concepts - essentially the idea that the variation is not entirely random, but is for example influenced by the creature's life and crucially that this makes evolution a more rapid process.
Most modern commentators feel these later theories have been thoroughly discredited. They may accept that life influences do affect variation, e.g. radiation increases the rate of mutation, they discount this since 'increased mutation rate is counter-survival'. They do not see a problem in the speed of evolution or in the development of 'extreme' perfection.
The details of Darwin's later ideas may well have been incorrect, but I think in light of his real concerns about the theory he was correct to look again at what is meant by 'random' variation.
Darwin understood that some kinds of change were more likely than others - without this negating the idea of changes being 'random'. Some lines of birds for example more often show variations in colouration than others.
This raises the possible of there being certain kinds of change, or certain mechanisms of change that accelerate evolution.
Mechanisms that accelerate evolution are actually central to evolution. Such mechanisms pay back not once, but many times over through the improved quality of the genes which they influence. In my view the evolutionary pressure on the indirectly acting mechanisms is every bit as intense or more intense than that on any one directly acting gene.