Humans have kept animals for companionship for thousands of years. Developments during the twentieth century in intercontinental transport, especially air travel, in the husbandry of animals, and in the technology for their housing; combined with widespread availability of electricity to power environment control systems (water filtration, oxygenation, heat lamps, humidifiers, photoperiod control etc), have made it possible to obtain and maintain a very much wider range of species of wild animals in captivity than ever before. Associated with this, there has been a huge increase in interest in keeping ornamental fish, amphibians, and reptiles, and the range of species of birds and mammals kept as pets has increased markedly also.