Organic Evolution

Theories of Evolution
Pre-Darwinism Period
According to Judas Christian the world was only a few thousand years old.The dominant Western view of the 17th and 18th centuries held that God and all his creations were locked in natural hierarchy.This hierarchy placed humans above rocks,plants and animals and below spiritual beings.Each creature in the chaing of beings is said to have been created separatelyby God's divine power.In the 18th century Carolus Linnaeus classified plants and animals.He placed humans in the same order as apes and monkeys viz primates.He did not suggest an evolutionary relationship between human and apes.He mostly accepted the notion that all species were created by God.But his classification scheme provided a background for the idea that humans,apes and monkeys had a common ancestor.According to Jean Lamarck the physical adaptations of an organism to its environment were transmitted to its offspring,the cumulative result being the development of new species after many generations.

Darwinism Period
In 1859 Charles Darwin published in the Origin of Species his theory backed by facts.His key idea was natural selection.He argued that all living beings tend to produce more offspring than can survive within the limitations of food and space available.Those offspring vary among themselves in many ways.Overpopulation leads to a struggle for existence.Variations result in some individuals being fit thus they live longer and produce more offspring and the advantages which contribute to survival are transmitted to the offspring.Thus progressive improvement of each species is achieved and a high degree of fitness is possessed by most of the surviving individuals.

Post Darwinism

Darwin's theory came for some critism particularly as a result of theory of genetics.Gregor Mendel experimented on garden peas.He found that when he mated long and short stemmed peas all the peas gave rise to long stems.The results gave rise to the notion that all the traits tall and short resulted from definite and different particles which persist from generation to generation.It can be inferred that each cell contain two pieces of genetic material giving the trait of length of the stem.In some the trait of long dominates the short which is recessive.Mendel's units of heredity are now called genes.He concluded that these units occured in pairs for each trait and that offspring inherited one unit of the pair from each parent.The genes of higher organisms are located on rod like bodies called chromosomes within the nucleus of the cell.Chromosomes also usually occur in pairs.Each gene carries a set of instructions encoded in its chemical structure.In 1953 Biologist James Watson and British chemist Francis Crick proposed that DNA is a long two stranded molecule shaped like a double helix.Of the various schools of evolutionary thought is the observation of mutations in plants by De Vries.He suggested that evolution arises through mutations instead of slight cumulative.