CONCLUSIONS 107 
Concerning the nature of the material basis of the | 
nerve impulse we can only say that it appears to involve 
that part of the chemical transformations in protoplasm 
which result in the production of carbon dioxide. 
Farther than this we cannot go at present. But it is 
certain that it has a chemical basis. Whether it has also 
a physical basis, such as a change in state of the colloidal 
substratum of the nerve, or not, we cannot yet say. 
Who shall write the chemical reaction of the future, 
embracing, not only the energy exchange, but the change 
in psychism as well? 
Finally, we come to the quantity of life, the point 
from which we started. The measure of this is the 
amount of respiration, or the amount of electrical 
response shown on stimulation.. The question of how 
much we are alive must be answered by the determina- 
tion of the extent to which we are undergoing energy 
transformation. Death and peace, life and struggle— 
these are the pairs which go together. The most perfect 
young life is that which shows the highest metabolic 
rate. We have shown the general correlation between 
the carbon dioxide production and the nerve impulse 
in its speed of propagation and ease of origin. There 
must, then, be a close correspondence between the habit 
of the organism and the general metabolic rate. The 
simile of the torch is obvious. The faster it burns the 
more light and life it has. The most vigorous life is 
that with the keenest chemical change. And this is also, 
as has been shown in another volume in this series, the 
criterion of youth. The most successful life is that in 
which the nervous system remains active, youthful, and 
alive for the greatest number of years. It is the youth- 
