As already noted, Dr. Boyd gives as the result of his observations on upwards of 500 human brains, that the weight of the left hemisphere almost invariably proved to be in excess of that of the right. In forty cases Dr. Broca found similar results, and observers of less note confirm them; so that but for the eminent authorities by whom those conclusions have been challenged, it would seem presumptuous to refuse them acceptance. But, countenanced by this conflict of opinion, it may be permissible to review the question in the narrower aspect of the present inquiry. Testing it then by a reverse process, and assuming hypothetically that the exceptionally dexterous right-handed man will be found to have the left hemisphere the heavier, and the true left-handed man _vice versâ_, the results arrived at by Dr. Boyd are altogether in excess of what might be anticipated. The number of the exceptionally dexterous right-handed, with an invincible instinctive preference for the use of that hand, though large in comparison with the no less dexterous left-handed, are nevertheless a minority. Habit, social usage, and education in all its forms, in the school, the drawing-room, the workshop, in all the arts of peace, and in nearly every operation of war, have so persistently fostered the development of the favourite hand that it is scarcely possible to arrive at any reliable statistics in proof of the initial proclivities of the large majority of conformists. Only a prolonged series of observations such as those already noted by Professor Baldwin, made at the first stage of life, and based on the voluntary and the unprompted actions of the child, can supply the needful data. But the careful observations of many years, prompted by a desire to master the source of an exceptional instinctive habit, have convinced me that the bias towards the preferential use of either hand in many, probably in the majority of cases, is slight. It is sufficient to lead to their following the practice of the determinately right-handed majority, but would not in itself present an obstacle to conformity to any prevalent usage, or to the influence of education. If then the preference of either hand furnishes any index of the relative development of the two cerebral hemispheres, what we may reasonably look for is a certain considerable percentage of brains with the weight of the left hemisphere in excess; a small percentage equally definitely characterised by the preponderance in the right hemisphere; while in the average brain the difference will be so slight as to be apt to escape observation. It is further to be noted that if the habitual use of the right hand tends in any degree to stimulate the development of the left cerebral hemisphere, then the examples of strongly-marked cases of such must greatly exceed those of the reverse type; since the left-handed man is almost invariably ambidextrous, and so subjects both hemispheres to the frequent stimulus of efferent nerve-force.