Several theories have been put forward to explain how and why animals spread from their place of origin to where they are now found, sometimes travelling thousands of kilometres and crossing barriers in the process. Theory of continental drift and existence of corridors and filter bridges between two land masses also explain distribution of animals and have been dealt with earlier. In addition to these the following theories explain the distribution of plants and animals across continents.
1. Brown’s theory of centrifugal speciation. It has been observed that largest populations of a species exist in the most favourable areas. Due to increasing numbers in these areas a population pressure may build up forcing some pioneers to venture into the peripheral areas which may not be as habitable. This will lead to isolation and speciation that will proceed from the centre to periphery due to the centrifugal force generated by the population pressure of the species. Such migrations lead to differentiation of races, clines or subspecies by way of character displacement across the range of distribution.
2. Age and area theory of Willis. J. C. Willis (1949) gave this theory based on his studies on plant geography. Plants have a tendency to enlarge their range of distribution slowly in all directions and hence the age of a species must be proportional to the area occupied by it at present. This means that widely distributed species are older as they should have taken long time to spread to such distant places and the endemic species must be young.
The species get subdivided into races and subspecies by isolation, mutations and other accumulating genetic changes. This theory presumes that all species spread at the same slow rate and at all time in the history of evolution. However, Fernald has pointed out that many plant species have spread rapidly while others slowly or have not extended their range.
For example when the glaciers receded from North America, the flora recolonised the vacated area that measured almost one-fourth of the continent within a span of only 25,000 years. Also, endemism is not always due to younger age of the species but also due to extinction of the species from all areas except small habitable localities left surrounded by barriers.
3. Climate and evolution theory of Matthew. W.D. Matthew (1939) being a geologist and mammalian palaeontologist believed in the permanency of continents and ocean basins. He thought that continental drift, corridors and filter bridges played no role in the distribution of animals. However, he believed that climate of the world changed throughout the history of evolution.
During warmer and humid periods shallow seas separated the continents and even the great land masses of northern continents were habitable where animals could freely migrate and evolve. But during cooler climates of ice ages, northern land masses became inhospitable and forced animals to migrate to the southern continents or perish.
Matthew, being a mammalian palaeontologist, thought that mammalian evolution supported this view that major placental mammals such as elephants and mammoths, camels, horses, carnivores and primates originated in the northern regions and then migrated southwards, while primitive monotremes and marsupials remained confined to the southern continents of Australia and South America.
P.J. Darlington (1957) based on his studies on zoogeography, although generally agreeing to the permanency of continents, came across evidence that major groups of vertebrates originated in the tropics and subtropics of the old world, which still retain the most abundant and varied fauna.