644 



SCIENCE. 



[N. S. Vol. XIV. No. 356. 



Leaving out of count the asymmetry in 

 the arrangement of the convolutions in the 

 two hemispheres, which cannot by any 

 amount of ingenuity be twisted into such a 

 form as to give a structural superiority to 

 one side more than the other, the only 

 marked difference which appears to possess 

 any degree of constancy is the increase in the 

 territory of the left parietal lobe produced 

 by the more marked depression of its lower 

 frontier line (Sylvian fissure). That this 

 is in any way associated with right-handed- 

 ness or with the localization of the active 

 speech center in the left hemisphere I am 

 not prepared to urge, because the same con- 

 dition is present in the ape. It is true that 

 some authorities* hold that the ape is 

 right-handed as well as man, but in the 

 gardens of the Royal Zoological Society of 

 Ireland I have had a long and intimate ex- 

 perience of both anthropoid and lower apes, 

 and I have never been able to satisfy my- 

 self that they show any decided preference 

 for the use of one arm more than the other. 



That differences do exist in the more in- 

 timate structural details of the two hemi- 

 spheres, which give to the left its functional 

 superiority, there cannot be a doubt ; but 

 these have still to be discovered. Bastian 

 has stated that the gray cortex on the left 

 side has a higher specific gravity, but this 

 statement has not as yet received corrobora- 

 tion at the hands of other observers. 



I have already mentioned that man's 

 special endowment, the faculty of speech, 

 is associated with striking changes in that 

 part of the cerebral surface in which the 

 motor center for articulate speech is lo- 

 cated. It is questionable whether the ac- 

 quisition of any other system of associated 

 muscular movements has been accompanied 

 by a more evident cortical change. The 

 center in question is placed in the lower 



*Ogle, 'On Dextral Preeminence,' Trans. Med. 

 Chirurg. Soc, 1871; Aim6 Pere, ' Les Courbures lat- 

 4rales normales an rachis humain,' Toulouse, 1900. 



and back part of the frontal lobe. We 

 have seen that the insular district is cov- 

 ered over in the hinder three-fourths of its 

 extent by the fronto-parietal and temporal 

 opercula, and thus submerged below the 

 surface and hidden from view. The brain 

 of the ape and also of the microcephalic 

 idiot with defective speech goes no further 

 in its development. The front part of the 

 insular district remains uncovered and ex- 

 posed to view on the surface of the cere- 

 brum. In man, however, two additional 

 opercula grow out and ultimately cover 

 over the fore part of the insula. These 

 opercula belong to the lower and back part 

 of the frontal lobe, and are to be looked 

 upon as being more or less directly called 

 into evidence in connection with the acqui- 

 sition of articulate speech. 



The active speech center is placed in the 

 left cerebral hemisphere. We speak from 

 the left side of the brain, and yet when the 

 corresponding region * on the right side is 

 examined it is found to go through the 

 same developmental steps. 



The stimulus which must have been given 

 to general cerebral growth in the associa- 

 tion areas by the gradual acquisition of 

 speech can hardly be exaggerated. 



During the whole course of his evolution 

 there is no possession which man has con- 

 trived to acquire which has exercised a 

 stronger influence on his higher develop- 

 ment than the power of articulate speech. 

 This priceless gift, ' the most human mani- 

 festation of humanity' (Huxley), was 

 not obtained through the exertions of any 

 one individual or group of individuals. It 

 is the result of a slow process of natural 

 growth, and there is no race, no matter how 



* Eudinger and others have tried on verj- unsub- 

 stantial grounds to prove that there is a difference in 

 this region on the two sides of the brain. There is, 

 of course, as a rule, marked asymmetry ; but I do 

 not think that it can be said vrith truth that the cor- 

 tical development of the region is greater on the left 

 side than on the right. 



