Darwin's Demon

Just as Laplace illustrated the nature of the Newtonian mechanics in terms of his "demon", Darwin introduced his "demon" (without this name, though), in order to illustrate the operation of natural selection; or rather, Darwin appealed to this demon in order to help the reader (Darwin's contemporary reader, in the Victorian age), in all probability.

if a being infinitely more sagacious than man (not an ominiscient creator) during thousands and thousands of years were to select all the variations which tended towards certain ends, for instance, if he foresaw a canine animal would be better off, owing to the country producing more hares, if he were longer legged and keener sight---greyhound produced. (Sketch, sect. ii)

The same demon is more elaborated in the Essay (1844) again.

Let us then suppose that an organism by some chance ... arrives at a modern volcanic island in process of formation and not fully stocked with the most appropriate organisms; the new organism might readily gain a footing, ...

Let us now suppose a Being with penetration sufficient to perceive differences in the outer and innermost organization quite imperceptible to man, and with forethought extending over future centuries to watch with unerring care and select for any object the offspring of an organism produced under the foregoing circumstances: I can see no conceivable reason why he could not form a new race ... adapted to new ends. (Essay, ch. ii)

But that's not the end of Darwin's theory. Such a being should be able to do a lot; but what Darwin wishes to say is that the same amont of work can be done without such a being. That is why he continues the argument as follows:

In accordance with the plan by which this universe seems governed by the Creator, let us consider whether there exists any secondary means in the economy of nature by which the process of selection could go on adapting, nicely and wornderfully, organisms, if in ever so small a degree plastic, to diverse ends. I believe such secondary means do exist. (ibid.)

Thus, Darwin's demon seems to be introduced but only for dispensing with it later. How can he do this? Not quite, for in the Origin, the demon appears in the guise of Nature, Her!