What did Darwin say about microbes, and how did microbiology respond?

Although it is commonly assumed that Darwin had nothing to say about microbes, he did in fact say quite a lot. He included microbes in his Beagle studies of the geographical distribution of organisms and used microscopic organisms as explicit exemplars of how adaptation did not imply increasing complexity. Darwin often discussed microrganismal classification, origins and experimentation in his correspondence. But despite his interests in microbial phenomena, Darwin's impact on microbiological thinking of the late nineteenth century was negligible. This limited response may be connected to today's assumptions about Darwin's neglect of microbes.