Abstract.—The account was based on an expedition to Jugoslavian caves in the summer of 1937, The caves studied occur in the Cretaceous limestone system called Karst, and the features of this formation were illustrated by lantern‐slides– those of cave interiors being made from photographs kindly lent by the Cave Research Society of Ljubljana.Some typical cavernicolous forms (mainly of Crustacea and Insecta) were discussed, and the types considered in relation to the standard spelaeological problems of pigmentation, blindness, distribution, and adaptation. It was submitted that a working hypothesis, within the frame‐work of the theory of Natural Selection, could be put forward to account for the widespread lack of pigmentation.The annual flooding to which certain regions are subjected was emphasized as a factor capable of influencing the distribution and evolution of cave animals, and of contributing significantly to their food