Organic evolution of origin is the process by which organisms change their genetic make up in such a way as to favour the emergence of new species better adapted for their environment than their ancestors and therefore giving rise to higher (i.e. more physiologically or phylogenetically sophisticated) organisms from lower ones.
In 1842 Darwin summarised his thesis as follows- this later became known as Darwinism.
Overproduction. Plants and animals produce far more off-springs than can survive.
Variation. No two living things are exactly alike
Struggle for existence. Since more living things are produced than can survive, a sort of struggle goes on among them.
Survival of the fittest. (Natural Selection). Since there is endless variation, some organisms are better fitted for the struggle for existence than others.
Inheritance of acquired characteristics. Descendants of the “fit” organisms would inherit the qualities which made their ancestors fit, and gradually new species would arise, well-adapted to their environment and very different from their remote ancestors.
In the above five theses, Darwin postulated his scientific theory of evolution based on observations of living species in his journey to Galapagos Island (and others). It presupposes that species are transmutable and that earth is very very old to accommodate the billions of years needed for the experiment of bringing to existence new species from pre-existing ones and that life on earth is also equally very old indeed.
Check alternative model of Old earth-Young Life
Evolution of man from the higher apes