Darwinism Theory of natural selection:-

The theory of natural selection is based on the following factor:-

(i) Rapid multiplication:- All animals and plants tends to multiply in geometrical progression. Every species produces more offsprings than can be supported in a particular environment and can survive.

(ii) Limited environmental resources (Space and food):- Increase of population in animals and plants requires more space and food. The space in the universe remains constant. The ultimate source of food for all plants and also animals consists of CO₂, of the air and water and mineral salts of the soil. These material do not increase. The carrying capacity of environment, that is, the number of individuals it can support, does not allow its population to grow beyond the limit and equilibrium is reached. Hence, the population size remains nearly constant.

(iii) Struggle for existance:- Every individual makes efforts for fulfilment of its basic needs. This competition for the primary necessities of life is called struggle for existence.

(iv) Variation:- Variation is the law of nature. Every individual varies in some respects, such as size, shape, structure and behaviour, from other of its species.

(v) Natural selection (survival of the fittest):- In the struggle for existence, the individuals which have more favourable variations well enjoy a competitive advantage over others which have less favourable of unfavourable variations, and will survive and reproduce.

(vi) Inheritance of usefuI variation:- The individuals, after their selection by nature in the struggle for existence, pass on their useful variations to the next generation. Thus, the offspring of the fit individuals will also be fit.

(vii) Formation of new species:- In each generation, new favourable variations appear, and supplement the favourable variations inherited from the parents.