When amphibians left the water, they had a great many problems to face. In particular, their sense of smell became less acute. Fishes can perceive the odours of substances dissolved in water. In order to differentiate between smells on land, the pioneering amphibian had first to dissolve the odorous substances in some fluid present in its nose. Its olfactory receptors could not immediately adapt to the new conditions and its forebrain, receiving no information to digest, remained idle, as it were. This is, apparently, the reason why the forebrain in the amphibians assumed another function. It began to help in analyzing visual, auditory and, perhaps, many other stimuli. For the first time a division of the brain appeared which dealt with all sorts of information.
In mammals the brain developed particularly rapidly. First, it developed individual zones, which were not as yet strictly dillerentiated. Each zone was responsible for the analysis of a certain kind of stimulation — visual, auditory, olfactory, or skin irritation. Higher mammals developed small areas of so-called association cortex, which lay between the analyzing zones. These zones continually grew and progressed in the course of the further evolution of the brain. In apes and humans they occupy a large part of the surface of the cerebral hemispheres. It is not difficult to guess that it is these zones that perform the most involved, purely human mental functions.