Ferns and fernlike plants were so numerous that the period has been known as an age of ferns. Earlier knowledge of these forests was based on fossils of a fragmentary character from which an accurate association of parts could rarely be obtained. A general relationship with the ferns was apparent, but careful study of additional material has given us a rather different view of Carboniferous plant life and we note a highly diversified array of forms with many suggestions of modern tendencies. The flora as a whole may be regarded as highly specialized for the conditions which prevailed at the time and were not to continue through subsequent periods. Warm temperatures and abundant moisture were essential especially to spore-bearing types, and the cold, arid conditions of the next period put an end to many of the groups, or greatly reduced their prominence. This could be regarded equally well as an age of insects, for some of these invertebrates acquired the greatest size they have ever had, particularly the dragonflies with a wing-spread of more than two feet in one of the largest fossils so far discovered. Cockroaches numbering upwards of five hundred species have been named. Though large they are hardly to be regarded as giants, lengths of three or four inches being about the limit. Some of the insect types of today quite evidently existed among the inhabitants of Carboniferous forests, but it is apparent that there were also some antiquated forms which may have descended from the trilobites. Although some authorities regard this as the period in which insects originated, there are others who maintain that definite beginnings are not established so readily on present evidence. Spiders are believed to have made their appearance at this time. Four-footed vertebrates resembling salamanders were prominent animals of the Carboniferous swamps. At first adapted to a life in water and later to land conditions, they are known as amphibians, the name being based on the ability to live in two different kinds of environment. Common living representatives of this group are the toads and frogs, but these tailless forms are not known among fossils of the Paleozoic era and are almost unknown throughout the Mesozoic. The Age of Amphibians, as we apply that phrase, was definitely not an age of toads and frogs. These primitive land animals were of different types, ranging from much smaller sizes up to the length of a crocodile. Most of them had short legs, and feet which were suitable for locomotion upon land, but many of the creatures probably spent most of their lives in the water. Tails were usually high and flattened as if for swimming, sometimes long, at other times greatly shortened in proportion to the body. Heads were generally large, jaws long, and mouths wide.