3. Struggle for existence: Due to shortage of food, water and space, there is severe competition among the off spring for existence. Every individual has few basic requirements, such as food, space, water, mate to reproduce and protection from enemies. In order to achieve basic needs, organisms compete with each other and it is known as struggle for existence. The struggle for an individual can be of 3 types:

(i) Intraspecific Struggle: The competition of the individuals of the same species. For example, fight between two dogs for a piece of meat. War is another example of intraspecific struggle among different human.

(ii) Interspecific Struggle: The competition of the individuals of the different species. For example, tiger attacks on deer for food.

(iii) Environmental Struggle: Every individual struggle against the change in environment such as temperature, humidity, level of water, rain, climate etc.

4. Variation: Each and every individual varies in several aspects to other individual. Even the offsprings produced by parents also differ in many aspects. The two individuals can be different from each other in their behavior, color, size, strength etc.

5. Natural Selection: Due to variation among different individual, they struggle towards their existence with different potentials. The variation in an individual may allow him to survive and complete its life cycle comfortable. Wheras, if the variations are unfavorable, the individual will struggle against every odds and as a result it may not be able to complete the life cycle. For example, fast running deer has better chance to escape from the tiger compared to the slow runner. Another factor is ability to adopt into the changed environment. Both Darwin and Wallace, recognized the environment as the principal factor for natural selection. For example, plants with ability to hold more water and can be able to reduce loss of water will ultimately survive, despite the physical strength, height and other characters.

6. Inheritence of useful variations: The individual survived due to unique variation, mate and produces their off spring to complete their life-cycle. As a result, they transfer the useful variations to the next generations and allow the individual to multiply. Darwin believed that any variation which can help the individual to survive and help to favouable for struggle will be inherited. He considerd the variation which may be acquired or inheritable.