been a form of unilateral stimulation which would act to effect a structural change in one hemisphere over and above the other. But, apart from this, there is every reason to expect, quite independently of function, that two organs of such comparative separateness and independence of function would not remain exactly balanced in function; in short, spontaneous variations giving advantageous dextrality would inevitably arise and persist as soon as the habits of life were not such that more important functions, such as locomotion, tended to suppress them and restore bilateral equilibrium. There are, as far as I know, very few published observations of fact in regard to simian or animal dextrality.
It is likely, therefore, that right-handedness in the child is due to differences in the two half-brains, reached at an early stage in life, that the promise of it is inherited, and that the influences of infancy have little effect upon it. Yet, of course, regular habits of disuse or of the cultivation of the other hand may, as the child grows up, diminish or destroy the disparity between the two. And this inherited brain-onesidedness also accounts for the association of right-handedness and speech—the speech function being a further development of the same unilateral potency for movement found first in right or left handedness.
- It is on this point that I differ from Wilson, who claims that while some are naturally right or left handed, most people owe the peculiarity to education; the evidence, apart from my experiments, is well put by Mazel, loc. cit.
- I know only the assertion of Vierordt that parrots grasp and hold food with the left claw, that lions strike with the left paw, and his quotation from Livingstone i. e., "All animals are left-handed" (Vierordt, loc. cit., p. 428). Dr. W. Ogle reports observations on parrots and monkeys in Trans. Royal Med. and Chirur. Society, 1871. Dr. Ogle informs me in a private letter that the chimpanzee which recently died in the Zoölogical Garden in London was discovered by him to be left-handed. I have addressed a circular letter to some of the officials in zoological institutions here and abroad, and hope to gather some facts in this way. It is evident that on this theory of spontaneous variation any change which produced a permanent organic superiority of one hemisphere would be sufficient, and the view that the difference in the hemispheres is due to a better blood supply to the left hemisphere might thus have its justification. As a matter of fact, the arterial arrangements do seem to indicate a more direct blood supply to the left hemisphere (cf. the note of Dr. J. T. O'Connor, apropos of my experiments, in Science, xvi, 1890, p. 3a 1). It is an interesting inquiry whether this arterial arrangement is reversed in left-handed persons. Wilson cites two cases in which there was no such correspondence (loc. cit., p. 179).