Darwin began to realize that every species of living thing has the potential to increase geometrically (2, 4, 8, 16, 32, 64, 128, 256, 512, ...), yet this pattern of population growth does not happen in practice. Darwin explains why organisms do not increase geometrically. His reasons were the competition between animals, the limited amount of food, the climate, and epidemics. All organisms are bound together in the struggle for existence by complex relationships between each other. Also, the struggle for existence is greatest between organisms of the same species.