Explanations concerning the origin of obligate cavernicoles, or troglobites, have long been a source of controversy among biospeleologists. It is suggested here that troglophiles become troglobites as the result of an adaptive shift to an ecological niche which cannot occur in epigean communities. This adaptive shift is facilitated by preadaptation of the incipient troglobite through the development of genoclines in its troglophilic ancestral population. The shift to troglobitism is forced by competitive exclusion of the incipient troglobite from its original troglophilic niche by an invading species or community. The major changes of fauna produced in temperate latitudes by the climatic fluctuations of the Pleistocene have resulted in a particularly rich troglobitic fauna composed mainly of cryophilic relicts inhabiting temperate caves.