47 4 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 usdumb. But arace 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 
