There is hardly any field of natural science where development is so erratic as that of the physiology of the brain. It is true that perhaps no branch of science develops quite evenly. Very often the domination of old and deeply-rooted theories is so strong that they cannot easily be removed under the pressure of new facts, and consequently they may hamper the progress of science in the given field for many years. But generally the development of natural science is straightforward in the sense that its every stage is a logical consequence of the previous stages, and that particular lines of investigation tend to complement each other and eventually to merge into a single whole.