A combination of ecology and evolutionary biology has been one of the hallmarks of The American Naturalist since the introductory comments in its first issue, in which the editors placed the new “magazine” in the footsteps of Darwin (only 8 years after the publication of On the Origin of Species). Singling out a single article to celebrate the 150th anniversary of the journal is no easy task. However, for several reasons, I think that “Parasitic Bacteria and their Relation to Saprophytes,” by Theobald Smith (1887), nicely captures its spirit. First, this article is an illustration of how interwoven ecology and evolution are and why both need to be studied simultaneously. Second is the personality of the author, who was among the first to import evolutionary thinking into human and veterinary medicine. Third, more than a century later, this article, while rarely read, is still inspiring. Theobald Smith (1859–1934) may not be familiar to today’s audience of The American Naturalist (he surely was not when the article was published). Along with Pasteur, Koch, Lister, and their disciples in Europe, Smith was one of the founding fathers of microbiology (P. F. Clark, 1959, “Theobald Smith, Student of Disease (1859–1934),” Journal of the History of Medicine and Allied Sciences 14:490–514). Smith’s broad knowledge of the bacterial world was acquired from reading and from repeating Pasteur’s and Koch’s experiments. Smith’s professional trajectory is also unusual because, as a gifted student in the Albany, New York, public schools, he was primarily attracted to mathematics and teaching. According to Clark, Smith would say, not too seriously, that “his first choice of occupation would have been a tramp, second a musician or mathematician” (p. 510). After gradu-