There is another way of looking at the fundamental importance of gradualness for Darwin. His contemporaries, like many people still today, had a hard time believing that the human body and other such complex entities could conceivably have come into being through evolutionary means. If you think of the single-celled Amoeba as our remote ancestor as, until quite recently, it was fashionable to do - many people found it hard in their minds to bridge the gap between Amoeba and man. They found it inconceivable that from such simple beginnings something so complex could emerge. Darwin appealed to the idea of a gradual series of small steps as a means of overcoming this kind of incredulity. You may find it hard to imagine an Amoeba turning into a man, the argument runs; but you do not find it hard to imagine an Amoeba turning into a slightly different kind of Amoeba. From this it is not hard to imagine it turning into a slightly different kind of slightly different kind of . . ., and so on. This argument overcomes our incredulity only if we stress that there was an extremely large number of steps along the way, and only if each step is very tiny. Darwin was constantly battling against this source of incredulity, and he constantly made use of the same weapon: the emphasis on gradual, almost imperceptible change, spread out over countless generations.
