The living world has millions of species and how they came into being has been a great mystery. The Biblical theory of Special Creation which says that God created the whole world in seven days flat, particular type of living beings being created on a particular day. Science does not believe in such theories. Several attempts have been made to account for this vast variety of living beings prominent among which are:
Originally proposed by Charles Darwin in his book Origin of Species in 1859, this theory proposes that nature favours the continuatino of those specie which have better chances of survival in future. Animals having features favourable to survival get preference over others who do not have such faetures.This theory has by far the maximum number of followers.
This theory proposes that those organs which have some use for a particular animal are retained, while those which do not have any use for it, fall into disuse and are consequently lost in the next generation, after a number of such generations. He supports his theory with several examples including the giraffe, snake and vermiform appendix in humans. The giraffe, this theory says, did not have such a long neck in ancient times. Since it used to eat tree leaves and they were found at a great height, it had to raise its neck upwards to catch them. In this way, the neck became longer and still longer with each generation, and after millions of years, we see the giraffe with such a long neck.
Mutations are sudden, dramatic and irreversible biological changes in an organism due to changes in genetic structure. This theory believes that evolution is a sudden process of change, not a slow and continuous process as proposed by Darwin and Lamarck. A well-known example quoted is that of the Anchon sheep, which suddenly developed very short legs. Most modern biologists combine this theory with Darwin\'s to give a satisfactory explanation of biological evolution.