A Request For The Hive Mind: Did Darwin Write About Microbes?

Having just written a book all about E. coli, including its evolution, I came to wonder what Darwin thought about microbes. I've searched far and wide. I've looked in biographies, for example, and the awesome site Darwin Online. I have found only one reference--to viruses:

A particle of small-pox matter, so minute as to be borne by the wind, must multiply itself many thousandfold in a person thus inoculated; and so with the contagious matter of scarlet fever. It has recently been ascertained that a minute portion of the mucous discharge from an animal affected with rinderpest, if placed in the blood of a healthy ox, increases so fast that in a short space of time the whole mass of blood, weighing many pounds, is infected, and every small particle of that blood contains enough poison to give, within less than forty-eight hours, the disease to another animal.

The Variation of Animals and Plants Under Domestication (Google Books page)

If this is, in fact, the only passage Darwin did ever write, it's deliciously ironic. It comes in the midst of Darwin's description of a possible molecular mechanism of heredity. Darwin speculated that particles stream from the body's cells to the gonads, in order to collectively transmit an animal's traits to its offspring (a process he called pangenesis). He thought the fast replication of viruses bolstered his idea that such particles might exist.

In other words, Darwin's one nod to microbes comes in part of a doomed idea about heredity. He doesn't wonder if the viruses themselves are evolving. I suppose I should cut Darwin some slack, since the germ theory of disease was still a somewhat crazy idea at the time, and it was not clear that viruses or bacteria had the same patterns of heredity as animals and plants.

Today, however, scientists see Darwin's central insights about natural selection at work all the time in microscopic organisms. Yesterday I wrote in Slate about how a nasty strain of E. coli has evolved to become even nastier. It was based on this paper in the Proceedings of the National Academy of Sciences. Since then, the journal posted a new commentary to the original paper in which the authors write,

The work of Manning et al. (2) reminds us that evolution of microbial agents is an ongoing process. E. coli O157:H7 is the most notorious of several different types of pathogenic E. coli that are ''relentlessly evolving'' (7).

But perhaps I've missed some obscure passage Darwin scribbled on the back of a take-out menu. I've tried searching not just for obvious terms, but for Victorian ones too, and have come up dry. Anybody care to prove me wrong?

Update: Thanks for the feedback so far as of 3/20 evening. I ended up with a couple more hits, or near-hits at least, the most interesting of which deals with why there is so much "simple" life on Earth today. In the 1861 edition of the Origin of Species, Darwin addresses critics who think that if natural selection can produce more "advanced" forms of life, then there should no longer be any "lower" forms left. Indeed Lamarck ended up arguing that there was a built-in arrow to evolution towards higher forms. The simple life we see around us today is just the result of recent spontaneous generation. Darwin pointed out that this was unnecessary in his own theory:

On my theory the present existence of lowly organised productions offers no difficulty; for natural selection includes no necessary and universal law of advancement or development--it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life. And it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule--to an intestinal worm--or even to an earth-worm, to be highly organised? If it were no advantage, these forms would be left by natural selection unimproved or but little improved; and might remain for indefinite ages in their present little advanced condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state. But to suppose that most of the many now existing low forms have not in the least advanced since the first dawn of life would be rash; for every naturalist who has dissected some of the beings now ranked as very low in the scale, must have been struck with their really wondrous and beautiful organisation.

("Infusorian animacule" and "rhizopods" refer to certain kinds of protozoans.)