There is no one mode of reproduction that is universally superior in selective terms, but in many circumstances viviparity of various forms offers good protection from parasites and predators and permits flexibility in dealing with problems of reliability and economy in adverse circumstances. Variations on the theme in biology are enormous, ranging from trophic eggs to resorption of partly developed embryos in hard times or when they are too numerous for the mother to bring to term, but among the most profoundly advantageous features of viviparity are various forms of physiological support and protection of the embryo, such as thermoregulation and osmoregulation. Since the developing offspring remains within the mother's body, she becomes, in essence, a walking incubator, protecting the developing young from excessive heat, cold, drought, or flood. This offers powerful options for dealing with excessive changes in climate or when migration events expose populations to unfavourable temperatures or humidities. In squamate reptiles in particular, there is a correlation between high altitudes or latitudes, colder climates and the frequency of viviparity. The idea that the tendency to favour egg-retention selectively under cooler conditions arises from the thermoregulatory benefits, and that it consequently promotes the evolution of viviparity as an adaptation, is known as "the cold climate hypothesis".