According to Darwin's theory of survival of the fittest (Natural selection), the organisms which are provided with favourable variations would survive, because they are the fittest to face their surroundings, while the unfit are destroyed. Darwin considered that useful variations are transmitted to the offspring and appear more prominently in succeeding generations. After some generations these continuous and gradual variations in the possessor would be so distinct that they form a new species. This is proved by, the pesticide resistant insects that have the ability to survive as they have resistance genes and so they are selected by nature. Other insects lacking these genes will be killed by pesticides.