The bacterial flagellum is an example of what Michael Behe describes as an irreducibly complex system. In his book, Darwin's Black Box, he explains that such irreducibly complex systems could not have arisen by a gradual step-by-step Darwinian process.

Because the bacterial flagellum is necessarily composed of at least three parts -- a paddle,a rotor, and a motor -- it is irreducibly complex. Gradual evolution of the flagellum, like the cilium, therefore faces mammoth hurdles. (p.72)

Behe summarizes the structure of the bacterial flagellum in these terms:

Some bacteria boast a marvelous swimming device, the flagellum, which has no counterpart in more complex cells. In 1973 it was discovered that some bacteria swim by rotating their flagella. So the bacterial flagellum acts as a rotary propellor -- in contrast to the cilium, which acts more like an oar.

The structure of a flagellum is quite different from that of a cilium. The flagellum is a long, hairlike filament embedded in the cell membrane. The external filament consists of a single type of protein, called "flagellin." The flagellin filament is the paddle surface that contacts the the liquid during swimming. At the end of the flagellin filament near the surface of the cell, there is a bulge in the thickness of the flagellum. It is here that the filament attaches to the rotor drive. The attachment material is comprised of something called "hook protein." The filament of a bacterial flagellum, unlike a cilium, contains no motor protein; if it is broken off, the filament just floats stiffly in the water. Therefore the motor that rotates the filament-propellor must be located somewhere else. Experiments have demonstrated that it is located at the base of the flagellum, where electron microscopy shows several ring structures occur. The rotary nature of the flagellum has clear, unavoidable consequences ... (pp. 70-72)

The consequences Behe refers to are inferred by the nature of its irreducibly complex components, the discovery of which undermines a Darwinian explanation of origins. Behe concludes:

In summary, as biochemists have begun to examine apparently simple structures like cilia and flagella, they have discovered staggering complexity, with dozens or even hundreds of precisely tailored parts. It is very likely that many of the parts we have not considered here are required for any cilium to function in a cell. As the number of equired parts increases, the difficulty of gradually putting the system together skyrockets, and the likelihood of indirect scenarios plummets. Darwin looks more and more forlorn. New research on the roles of the auxiliary proteins cannot simplify the irreducibly complex syetem The intransigence of the problem cannot be alleviated; it will only get worse. Darwinian theory has given no explanation for the cilium or flagellum. The overwhelming complexity of the swimming systems push us to think it may never give an explanation. (p. 73)

Behe concludes that such irreducibly complex systems were ultimately the result of intelligent design. (It should be pointed out that Behe has no objections to the concept of universal common ancestry. His objections to evolution are limited to the rejection of the neo-Darwinian mechanism as a sufficient explanation for the origin of all biological systems.)