Animals are not uniformly distributed on land and in freshwater. They are restricted to certain places by several factors such as climate, food, shelter, flora and fauna etc. Generally distribution can be classified into three categories, namely, Continuous, Discontinuous and Bipolar distribution.
1. Continuous distribution. Eurytopic or wide-ranging animals are adapted to a wide variety of environmental conditions and are not specific to any particular type of food or may have special power to cross barriers either by flight, rafting or swimming and adapt to new environmental conditions. Wide ranging animals include rats, bats, hawks, cuckoos, cockroaches, flies, mosquitoes, lizards, snakes and man. Many of the eurytopic animals have continuous distribution because they are companions of man and have travelled to long distances with him.
2. Discontinuous distribution. When continuity of distribution of a species is broken by uninhabited areas which are sometimes very large stretches of oceans. There are primarily four reasons why animals are distributed in widely separated areas.
1. Animals reach distant areas by sweepstake routes, as insects, snails and rats by rafting, turtles by floating and swimming and birds blown by storms.
2. The species was earlier distributed continuously but the land masses in the intermediate areas submerged, breaking the species into widely separated populations.
3. Continental drift separated the continents and carried them to long distances, isolating the animals from other areas.
4. A widely distributed species can become extinct in the intermediate areas due to change in the climate to which the species is unable to adapt.
Examples:
Peripatus (Phylum Onychophora) has 75 species distributed in Southeast Asia, East Indies, Australia, New Zealand, Africa, South and Central America.
The three surviving genera of lungfishes occur in three different continents; Protopterus in Africa, Lepidosiren in South America and Neoceratodus in Australia.
Similarly the limbless amphibians (Apoda or Gymnophiona) of the genera Ichthyophis, Gegenophis and Dermophis occur in Africa, South America, Central America, South Asia and East Indies.
Ostriches are distributed in the southern hemisphere; Rhea americana in Argentina and Brazil, Struthio camelus in Africa, Emu in Australia, Cassowaries in North Queensland and New Guinea and Kiwi in forests of New Zealand.
Marsupials occur in Australia but one family Didelphidae occurs in South America that includes opossum (Didelphis), water opossum (Chironectes) and opossum rat (Coenolestes).
The common camel (Camelus dromidarius) occurs in the Middle East, India and Northern Africa; Camelus bactrianus is found in Mongolia and China, while two species, Llamavicuna and Llama guanaco are found in the mountainous regions of South America.
Of the two elephant species surviving today, Loxodonta africana occurs in Africa while another species, Elephas maximus is found in India, Burma, Thailand and Sri Lanka.
Alligators occur in America and China; Alligator mississipiensis in Americas, Caiman in South America and Alligator sinensis in China.
3. Bipolar distribution. Some species because they are adapted to the cold arctic climate cannot migrate to the warmer areas and hence restricted to the Polar Regions. Polar bears, arctic fox, lemmings, coyote and reindeers are found in the northern Polar Regions while penguins are restricted to the Antarctica region. Among fishes, salmons migrate up to the temperate and polar tributaries for breeding.
Other polar species are Coelenterates (Lampra, Myriothela, Grammaria, Ptychogastria, Botrynema, Sardina). Mollusca (Limacina, Clione). Urochordata (Didemnus), an ascidian.