During the Cambrian period major land masses occurred south of equator and Africa, South America, Australia, Antarctica and parts of Asia formed a single continental mass, Gondwanaland, that extended up to the South Pole but the position of continents was reverse of what it is today. There are evidences of major mountain building in North America and Europe and of glaciation during Devonian period, when much of Gondwanaland was covered with ice sheets. There was sharp decrease in the atmospheric CO during this period perhaps because of invasion of land by plants. Invasion of land by 2 plants attracted invertebrates and then vertebrates on land, pioneers among them must have been detrivores rather than plant feeders, such as springtails, millipedes and mites. By Carboniferous period even the northern land masses started to move southwards to join Gondwanaland, forming a single supercontinent called Pangaea towards the end of this period. Most of the present day coal beds were formed when these forests were buried during this period (carboniferous means coal-bearing). High levels of atmospheric oxygen, low levels of CO and 2 abundance of food enabled invertebrates to diversify and become giants. Pangaea continued to exist till Permian when it again broke and started to drift apart. During Permian, about 270 million years ago, a single large continent Pangaea started to split into two land masses, the northern Laurasia and southern Gondwanaland, separated by the sea of Tethys. The splitting perhaps caused what is known as the worst mass extinction in the history of evolution. During Jurassic, the continental masses began to break up leading to the formation of Atlantic Ocean. Americas began to drift westward, Antarctica and Australia southwards and India towards the northeast. North and South Americas were connected together by a narrow corridor, through which migrated reptiles and primitive mammals. Towards the end of Cretaceous some catastrophe struck the earth resulting in another mass extinction and extermination of the most powerful of all animals that ever existed – the dinosaurs. During Eocene, North and South Americas got disconnected by the submergence of panama connection, isolating the South American continents from the others for a very long time. Almost at the same time North America also got disconnected from Europe for ever, although a filter bridge may have existed between the two continents for some time. During Miocene Indian plate had moved sufficiently northwards to collide with the Eurasian plate, obliterating the Sea of Tethys and starting a process of mountain-building that continued well into the Pliocene. In the beginning, the rising Himalayas did not produce an effective barrier between the Palaearctic and Oriental Regions but they gradually rose to great heights of today and effectively checked the movement of animals between the two regions. Sclater (1857) was the first one to give concepts of zoogeography and divided the continental masses into six realms based on his studies on the bird fauna under two Creatio or centres of Creation, namely, Palaeogeana (Old world) and Neogeana (New world). The widely accepted modern classification of land masses into regions is given below which is based on Wallace (1876) and Darlington (1957).

- 1. Realm MEGAGEA

a. Region PALEARCTIC (Europe, Russia, Mediterranean). b. NEARCTIC (North America up to the middle of Mexico).