the pseudosuchia lineage of thecodont reptiles produced birds which invaded a new environment of flying in the air, while another branch of thecodont reptiles gave rise to mammals. Tachyschizia can also be seen in Darwin’s finches which have all branched off from a single population that was blown off South American coast to the Galapagos Islands, where ecological niches were vacant and natural selection relaxed.

## Horoschizia

This is moderate rate of splitting of phyletic lines under moderately changing environmental conditions when natural selection is not too harsh. Splitting takes place after long intervals.

## Bradyschizia

A lineage persists without splitting for a long time and the evolution is slow, but a split can happen after a long gap and some branches may survive while the others become extinct. The rate of speciation can be measured as an average of several branching lineages, which would include both surviving and extinct ones. Cladograms that show branching lineages are developed with the help of computers and compared with actual phylogenies to arrive at a realistic conclusion.

## CONTINENTAL DRIFT

The theory that the present day position of continents is not permanent but the continents constantly drift and change positions, was first proposed by Alfred Wegener who from close studies of maps, discovered that coastlines of all continents fitted together like a jigsaw puzzle, suggesting that all continents in the past were placed together in a mass of supercontinent. Wegener believed that continents were made of light material called Sial (made of silica and aluminium) that floated over a heavier material Sima (made of silica and Magnesium) that makes ocean floors, like icebergs floating in ocean water. Later, in 1937, A.L. Du Toit elaborated on this theory in his book, On Wandering Continents, which explained many of the puzzles of discontinuous distribution of animals. But it was much later in the late 1960s that detailed studies of ocean floors revealed that the earth’s crust, which is made of hard rocks and soil, is only 30-60 km thick and is broken into several plates that float over the molten mass of the interior of earth that is constantly in circulation. The theory is now known as plate tectonic theory which postulates that the continental plates move at a very slow pace to change positions that can be measured in thousands of kilometres. The innermost part of earth is called Barysphere or core or nife which is about 4,400 km in diameter, surrounding which is Pyrosphere or mantle, which is 2,500 km thick molten mass and like a wrapper is a thin crust or Lithosphere of only 30-60 km thickness that is made of rocks and soil and which supports life on its surface. Continents move because the crust is broken into blocks or plates that float on the hot molten lava of mantle that keeps in circulation due to slow convection currents produced by the heat emanating from barysphere or core. Due to this heat the molten basalt rises upwards and spreads horizontally just under the crust and cools down. There are subduction zones where the crust sinks down and melts into the mantle due to the pressure of spreading sea floor. This process of spreading and sinking of the earth’s crust makes the continental plates move.