PROFESSO R VIRUHO W AND E VOL UTION. 629 
That "matter is not itself alive" Professor Knight 
seems to regard as an axiomatic truth. Let us place in 
contrast with this the notion entertained by the philosopher 
Uebervveg, one of the subtlest heads that Germany lias 
produced. " What occurs in the brain/' says Ueberweg, 
" would, in my opinion, not be possible, if the process 
which here appears in its greatest concentration did not 
obtain generally, only in a vastly diminished degree. Take 
a pair of mice and a cask of flour. By copious nourish- 
ment the animals increase and multiply, and in the same 
proportion sensations and feelings augment. The quantity 
of these latter possessed by the first pair, is not simply 
diffused among their descendants, for in that case the bust 
must feel more feebly than the first. The sensations and 
feelings must necessarily be referred back to the flour, 
where they exist, weak and pale it is true, and not concen- 
trated as they are in the brain."* We may not be able to 
taste or smell alcohol in a tub of fermented cherries, but 
by distillation we obtain from them concentrated Kirsch- 
wasser. Hence Ueberweg's comparison of the brain to a 
still, which concentrates the sensation and feeling, pre- 
existing, but diluted in the food. 
" Definitions," says Mr. Holyoake,f "grow as the horizon 
of experience expands. They are not inventions, but 
descriptions of the state of a question. No man sees all 
through a discovery at once." Thus Descartes' notion of 
matter, and his explanation of motion, would be put aside 
as trivial by a physiologist or a crystal lographer of the 
present day. They are not descriptions of the state of the 
question. And yet a desire sometimes shows itself in dis- 
tinguished quarters to bind us down to conceptions which 
passed muster in the infancy of knowledge, but which are 
wholly incompatible with our present enlightenment. Mr. 
Martineau, I think, errs when he seeks to hold me to views 
enunciated by " Democritus and the mathematicians." 
That definitions should change as knowledge advances is in 
accordance both with sound sense and scientific practice. 
When, for example, the undulatory theory was started, it 
was not imagined that the vibrations of light could be 
* Letter to Lange: " Gesckiclite des Materialisrous," Bweite Aufl., 
vol. ii., p. 521. 
f Nineteenth Century, September, 1878. 
