Struggle for existence: According to Darwin, individuals multiply in geometric ratio, but the space and food remain constant. This leads to competition for getting requirements of life. This competition is called Struggle for existence. The struggle may be intra-specific, inter-specific or environmental.

i. Intra-specific struggle:

It is the competition among the individuals of the same species.

This type of struggle is very severe, because the need and approach of all competing organisms is precisely same. e.g. struggle between cow and cow, horse and horse, deer and deer, etc. for getting grass.

ii. Inter-specific struggle:

It is the struggle between the organisms of different species living together. Individuals of one species compete with other species for food, shelter and breeding place. e.g. Struggle between cow, horse nd deer for getting grass.

iii. Environmental struggle:

It is the struggle of all living forms against adverse environmental conditions, i.e. against natural calamities like extreme heat or cold, drought, storms, earthquakes, volcanic eruptions etc.