It is necessary at the outset to define the limits of this paper. For those profound disorders of mind to which imbecility and idiocy belong, sufficient causes have been found in neural degeneration or acquired disease. Although the fundamental cyto-architectonics of these cerebral degenerates has not been clearly worked out, we can separate off, as a result of post-mortem findings, a group of cases, both congenitally and acquired, which belong to neurology rather than to psychology. Admitting the fact that both imbeciles and idiots have exhibited disturbances of behaviour and emotional control, there is no reason to deny that such phenomena are secondary to neural defects and to such glandular disorders as accompany these neural defects, or occur concurrently as indication of general congenital inferiority. Furthermore, a class of defectives of high grade can be marked off in whom the primary defect is glandular, and in whom the psychological shortcomings are also to be regarded as secondary. But when we turn to that considerable group of children with intelligence quotients of about 80%, the problem of cause assumes a different complexion. Cases that fall within the statutory limits can be enumerated because they require special supervision and control, or, at least, special educational treatment; but of those children who cannot be dealt with legally, or who merely pass muster within the school as dullards, very little can be said as regards their number and distribution, and unless they die of intercurrent illnesses and are brought to post-mortem, nothing can be stated as to the presence of cerebral degeneration, however minor in degree.