In 1880, Herbert Spencer postulated that fertility increases in response to the progress of society and that in fact is a great agency for mankind’s advancement.
He, however, went on to add that when population growth becomes excessive and threatens the very existence of the human species, individuation steps in and resolves all these difficulties.
Another biological theory was put forward by an Italian biologist, Corrado Gini in recent times. This theory, known as the cyclical theory, states, that, population tends to follow an evolution which is similar to that of the life cycle of the individual passing rough the successive stages of development, maturation and involution.