How did Darwin explain fitness of organisms ?

Explanation

The organisms struggle for food, space and mate. These are the limiting factors and competitionexists among organisms of the population. Darwin denoted the struggle exist between different groups in a population.
(i) Intraspecific struggle for food, shelter and mate.
(ii) Interspecific organisms struggle for food and shelter.
(iii) Struggle with environment to cope up with climatic variations like flood, earthquakes, drought etc.
In these struggles, Darwin proposed that the fittest will overcome the struggles and survive and others will perish.