ANCIENT ANIMALS 435 



of the terrestrial world with the vertebrates. The origin of insects remains 

 a mystery. The oldest fossils are those of primitive wingless forms (Collem- 

 bola, or springtails) found in association with land plants (psilophytes) 

 of early Devonian age. Then comes a great gap, until suddenly winged 

 insects appear in abundance in the coal measures. How and when they 

 acquired wings is completely unknown. 



Many of the coal forest insects belonged to a primitive stock (Paleodic- 

 tyoptera) which is apparently ancestral to all other winged insects. They 

 had large net-veined wings which were outstretched at rest, like those of 

 modern dragonflies. Another abundant group included the ancestral 

 cockroaches, which differed only in minor respects from those of today. 

 Very large insects occurred in the coal forests; one dragonflylike form had 

 a wingspread of about 2 feet. There were, however, many small species 

 as well, especially among the cockroaches, and if one includes the minute 

 wingless springtails and similar forms, the average size of the Carboni- 

 ferous insects was probably not much greater than in the modern fauna. 

 None of these ancient insects had a pupal stage in the life history, and 

 none of the modern insect orders was yet in existence during the Carbon- 

 iferous period. 



The end of the era of ancient life. The Appalachian revolution that 

 closed the Paleozoic era was a time of profound changes in life, as we have 

 pointed out in discussing the history of plants. Many ancient groups of 

 animals died out during the Permian and were replaced by new and better 

 adapted types, both in the seas and on the lands. Continental uplift 

 caused withdrawal of the wide interior seas, and in the remaining narrow 

 marginal seas crowding, competition, and lowered temperatures killed 

 off a host of Paleozoic groups. The last trilobites disappeared; the brachi- 

 opods, sea lilies, old types of corals, and other invertebrates that had 

 once been dominant were reduced to unimportant remnants; and the 

 cephalopod molluscs, which had flourished since the Ordovician, died 

 out, except for a few survivors, from which the dominant ammonites of 

 the Mesozoic era were destined to develop. 



On the lands the effects of the physical and climatic changes of Permian 

 time were even more profound. The swamp forests disappeared, and with 

 them most of their characteristic animal inhabitants. The ancient groups 

 of winged insects were supplanted by new types, some of which were the 

 ancestors of the modern insect orders with complete metamorphosis. In 

 these the larva is very different from the adult insect, and there is an 

 inactive pupal stage interpolated in the life history (Fig. 24.10); within 

 the pupal case the larval body is transformed into that of the adult. This 

 "invention" permitted the evolution of larval forms adapted to different 

 environments and foods than those required by the adults and enabled 

 insects to endure periods of drought or cold in the nonfeeding, quiescent 



