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 Lamarc
Cavefish: Retrospective and Prospective. Biodiversity and Distribution of the Subterranean Fishes of the World. Conservation of Subterranean Fishes. Behavioral Patterns in Subterranean Fishes. The Evolutionary Genetics of Cave Fishes: Convergence, Adaptation and Pleiotropy. Development as an Evolutionary Process in Astyanax Cavefishes. Subterranean Fishes of North America: Amblyopsidae. Subterranean Fishes of Mexico (Poecilia mexicana, Poeciliidae). Subterranean Fishes of Brazil. Subterranean Fishes of Africa. Subterranean Fishes of China. Subterranean Fishes of India.