This evolutionary model, unlike most 19th century evolutionary theories, is cyclical, beginning with the differentiation and increasing complication of an organic or "super-organic" (Spencer's term for a social system) body, followed by a fluctuating state of equilibrium and disequilibrium (or a state of adjustment and adaptation), and, finally, the stage of disintegration or dissolution. Following Thomas Malthus' population principles, Spencer concluded that society is constantly facing selection pressures (internal and external) that force it to adapt its internal structure through differentiation.