How did Darwin explain fitness of organisms ?

Explanation

Darwin.s theory was based on several facts, observations and influences. They are:
1. Over production (or) prodigality of production
All living organisms increase their population in larger number.
2. Struggle for existence
Organisms struggle for food, space and mate. As these become a limiting factor, competition exists among the members of the population.
Darwin denoted struggle for existence in three ways-
(i) Intra specific struggle between the same species for food, space and mate.
(ii) Inter specific struggle with different species for food and space.
(iii) Struggle with the environment to cope with the climatic variations, flood, earthquakes, drought, etc.,
3. Universal occurrence of variations
(i) No two individuals are alike. There are variations even in identical twins.
(ii) The useful variations found in an organism help them to overcome struggle and such variations are passed on to the next generation.
4. Origin of species by Natural Selection
(i) According to Darwin, nature is the most powerful selective force.
(ii) He compared origin of species by natural selection to a small isolated group.
(iii) Darwin believed that the struggle for existence resulted in the survival of the fittest.
(iv) Such organisms become better adapted to the changed environment.