In most habitats, adaptations are the single most obvious aspects of an organism's phenotype. However, the most obvious feature of many subterranean animals are losses, not adaptations. Even Darwin saw subterranean animals as degenerates: examples of eyelessness and loss of structure in general. For him, the explanation was a straightforward Lamarckian one, and one that did not involve adaptation and the struggle of existence. This volume is a comprehensive account of all known species of subterranean fishes. It includes an extensive introduction, history of investigations, consideration of non-stygobitic fishes in caves, and detailed analysis of the conservation status of these very rare animals.