The maximum capacity of the mind in any particular field is, therefore, practically determined by physical inheritance of an abundant supply of nerve endings. Thus, it may be that one individual is born with two or three times as many nerve terminals connecting at the point at the back of the eyeball where the optic nerve—which is simply a bundle or rope of nerve fibres—is attached to the mechanical apparatus upon which the reflection of objects passing before the field of vision is registered. Such an individual’s powers of observation are normally greater than those of the person of less fortunate heredity in this respect, whose lesser number of terminals of the optic nerve fibres limit his powers of optical perception and observation. Thus, one person may see at a glance a hundred details, all of which register sharply upon his consciousness, while another sees only the gross outlines and shadows, and in between is the average person who sees some details but not all. It is well known to psychologists and biologists, although not generally understood by those who have not made a special study of these sciences, that mental _capacity_ does not change or increase materially after the individual has reached maturity. It may be diminished through accident or disease, but the chief increase in adult life is in the volume and variety of stored-up impressions. The average girl of eighteen or boy of twenty has reached the approximate limit of his or her mental capacity. The mental tank will never grow much larger. It may be half empty or almost entirely vacant, but unless at the average age of university sophomores scientific mental tests prove the individual to be possessed of average or better than average mental capacity, it is futile to expect any great intellectual development to take place in later life. But while the maximum capacity of the mind depends upon physical inheritance, the actual ability which is necessarily reflected in the scores made by a person subjected to mental tests is determined by the number and variety of nerve connections that have actually been made by environment or training. Inheritance sets the maximum limit, but as a matter of practice this maximum is never reached, or at least is so seldom reached by any individual that it can hardly be said of any human being that he has developed his mind in any direction to the utmost limit of its capacity. What we actually measure in scientific mental tests is a complex of natural or inherent abilities plus the results of education and training; because, while it is possible to a considerable extent to eliminate by properly devised tests a record of the individual’s acquired knowledge, it is practically impossible to distinguish between acquired and inherent mental ability.