The study of the dreams of the blind thus emphasizes many points of interest in the nature and development of the cortical centres of the human brain; it graphically illustrates the explanatory power of the modern view of their functions; and it presents in a new aspect certain characteristics of their constitution. It shows beyond a question that the power of apperceiving sight-images is in no true sense innate, but is the product of slow development and long training. That the same holds true of other centres is proved by a mass of evidence gathered from many quarters; with regard to the motor centres, it is even experimentally determined by the observation that stimulation of the central convolutions of the brains of puppies fails to excite the appropriate movements of the legs, unless the puppies are already nine or ten days old. These facts will be utilized in the formulation of an important developmental law applicable alike to physiological and to psychological processes.
The "critical period," revealed by the above research, must not be understood as marking the point at which the visual centre begins its life; this indeed occurs at a much earlier age, and this centre from the outset and continuously increases in complexity and stability. Nor was the statement made that there was no difference here relevant, between the loss of vision at different ages before the critical period. That a child who has seen up to the fourth, or the third, or even the second year of life, probably retains some traces of visualizing not attainable by those who attendedthe school of vision for a shorter time or not at all, is believed on evidence of a general, but not as yet of a specific nature. Among other facts it is indicated by the influence of the age of blinding on the future development of noted blind persons. Similarly, after the critical period, the same processes of growth and assimilation continue, as is evidenced by the vague character and comparatively early decay of the dream-vision of those becoming blind close upon the end of the seventh year. The more time spent in gathering in the provisions, the longer do they hold out. The significance of the critical period lies in its demonstrating a point in the growth of the higher sense-centres, at which a divorce from sense-impression is no longer followed by a loss of their psychical meaning; a point at which imagination and abstraction find a sufficiently extended and firmly knit collection of experiences to enable them to build up and keep alive their important functions; a point where the scholar dispenses with the object-lesson and lives off his capital; a point at which the scaffolding may be torn down and the edifice will stand.
The indication of such a period in the development of the human mind brings clearly into view the dependence of the higher mental processes upon the basis furnished them by the experiences of sensation; it strongly suggests a rational order and proportion in the training of the several faculties of the child's mind; and finally, it prevents the formation and survival of false notions, by substituting certain definite though incomplete knowledge for much indefinite though very systematic speculation.
The Riverside PressElectrotyped and printed by H. O. Houghton & Co.Cambridge, Mass., U. S. A.