e for existence ↓↓↓↓➡According to the theories of Darwin , individuals multiply in geometric progression. EXAMPLE : 2 , 4 , 8 , 10 .....➡However food multiplies in arithmetic progression .Example : 2 , 4 , 6 , 8 ........➡ Space for accommodation of ORGANISMS is constant and does not increase .➡This leads to intense STRUGGLE between living organisms for food , shelter, climate , breeding places , food supply and mate .➡ Intense competition arises and the competition is of different levels such as :(i) interspecific ( different species ) and(ii) intraspecific ( same species ) .➡During this process of struggle for existence , only the strongest individuals who are fit for the ENVIRONMENT survive.➡The ones who are strong is only selected by nature and the rest die . This is called Natural Selection.➡The process of struggle for existence while facing competitions within or between other species is called Struggle for existence .Explanation with example :➡Oysters lay 60 to 80 million eggs every spawn . If all would have survived,  then the oysters would have filled up the entire Earth.➡Same with bacteria and paramecium .➡Elephants who are the slowest breeder breeds once in 15 years average.If all survived , the there would have been trillions of elephants up to date.Thus natural selection and struggle for existence leads to VARIATION and evolution of mankind !