474 THE CAUSES OF THE xi 



time, that it may depend upon structural differ 

 ences which shall be absolutely inappreciable to 

 us with our present means of investigation. What 

 is this very speech that we are talking about ? I 

 am speaking to you at this moment, but if you 

 were to alter, in the minutest degree, the propor 

 tion of the nervous forces now active in the two 

 nerves which supply the muscles of my glottis, I 

 should become suddenly dumb. The voice is pro 

 duced only so long as the vocal chords are parallel ; 

 and these are parallel only so long as certain 

 muscles contract with exact equality ; and that 

 again depends on the equality of action of 

 those two nerves I spoke of. So that a change of 

 the minutest kind in the structure of one of these 

 nerves, or in the structure of the part in which it 

 originates, or of the supply of blood to that part, 

 or of one of the muscles to which it is distributed, 

 might render all of us dumb. But a race of dumb 

 men, deprived of all communication with those 

 who could speak, would be little indeed removed 

 from the brutes. And the moral and intellectual 

 difference between them and ourselves would be 

 practically infinite, though the naturalist should 

 not be able to find a single shadow of even specific 

 structural difference. 



But let me dismiss this question now, and, in 

 conclusion, let me say that you may go away with 

 it as my mature conviction, that Mr. Darwin s 

 work is the greatest contribution which has been 



