 MacLean proposed that as animals evolved over the hundreds of millions of years, in order for an increased chance of survival, higher order animals developed an increased cognitive ability, which resulted in an increase in brain size.MacLean firmly believed that the driving force in the development of the neocortex was the development of social behaviours, such as the separation cry between infant and mother during the development phase of offspring.It followed the idea that mammals evolved through learning about different methods of survival, as these mammals learnt various methods of survival through particular encounters, their brains developed into far more complex cytoarchitectures.