Darwin begins this section by looking at how different species are created. Minor variations become dominate or fade away as species develop through the “struggle for life” (p51). By this he means that if the variation is helpful to the individual in relation to their own environment and in relation to it’s peers, then it will help them live and be passed on to their offspring.
“I have called this principal, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark it’s relation to man’s power of selection” (p52)
Others call this by the more descriptive and at times more accurate “Survival of the Fittest” (p52). Darwin points out how, when gaze upon nature’s beauty and abundance that it is, in birds, insects plants, all manner of beings, a struggle for survival. He clarifies that specifically what is important is survival of an individual and leaving surviving offspring.
All organisms, from plants which may produce hundreds of seeds to slow breeders such as a human or an elephant, create more offspring than will survive. If this were not the case, due to exponential growth there would be an overpopulation. This is seen in plants after two or three subsequent seasons of favorable conditions or when domestic animals are introduced to environments where they are without natural predators.
Darwin details instances where the competition for food or predators keep populations in check, but a more powerful factor can be climate and extended times of cold or drought. Even when climate varies in such a way which might seem more favorable for survival of a species,, one individual may have a variation which helps them prosper over the other and in turn crowd out the original for precious resources. When climates are extreme species tend to struggle against the environment, when they are more hospitable the struggle is between species.
In some cases where large species occupy a small tract of land, this abundance can be a detriment as it is prone to an epidemic. More often though, a large stock of individuals of a species is healthy for it’s preservation. Next Darwin discusses the interrelation of species; how the introduction or exclusion of one plant or animal can have far ranging and unexpected consequences on the survival of others. Darwin offers examples of this in the vegetation of wild and enclosed areas of British pastureland and in feral dogs and horses he observed in South America.