XIV INTRODUCTION. 
changes is related to consciousness so as to affect 
it. The next change, so variable in amount, due 
neither to vibrations nor to sense of sound but to 
the idea which the sound symbolizes, begins in the 
mind ; and science does not know how it happens 
that it is accompanied with change in the brain, of 
strictly proportionate amount. : 
These are undeniable facts, though not what a 
confiding public has been always taught in science 
lectures. It may be added that there is no ex- 
perimental evidence to show whether the amount 
of nervous change resulting from a given amount 
of physical stimulus is ever the same after passing 
through a nervous centre in which it affects con- 
sciousness as it would be after passing a centre in 
which it does not. But we have every reason to 
believe that the reflex actions in which no con- 
sciousness is involved are strictly proportional to 
the amount of stimulus ; while we know that when 
consciousness is affected it will often happen that 
no obvious action will follow a stimulus which 
would otherwise be sufficient to produce move- 
ments. 
In defending the position that the evolutions 
observable in organization are definite, I have 
used in the address which follows, few and simple 
illustrations, but I trust they are sufficient to give 
stability to the argument. Manifestly the more 
complex problems of morphology could not have 
