Most of the frogs and toads included in this volume belong to a number of small families—some with just one, two, or three species. Most of these small families are thought to have arisen early on in the history of frog evolution; some would have been more widespread in former times before they were replaced by more successful frogs that evolved later. However, others evolved in isolation and never had an opportunity to spread.
Following the accounts of these small families, this volume introduces some of the larger, more recently evolved families, including the rain frogs and the “true” toads. Frogs and toads are extremely diverse in appearance and habits. There are species that never leave the water and others that never enter it, even breeding on land. Some live only in waterfalls, while others spend nearly all their lives underground, emerging to breed only during a few nights of heavy rain each year.
The subject of conservation is a recurrent theme. Frogs are perhaps more vulnerable to extinction than any other group of animals, partly because their life cycles usually include both an aquatic and a terrestrial stage. They are unable to cope with pollution or habitat destruction and are susceptible to a number of other factors that are threatening their continued survival, some of which are still unclear. We do know, however, that many species have become extinct in recent times, and many more are likely to go the same way over the next few years.