1899] THE SCOPE OF NATURAL SELECTION 185 
as opposed to non-living substances therefore must be found, if it exist 
at all, in some other property of living matter, and it may possibly lie 
in the third feature that has been noticed, its power of maintaining a 
constant mass of unstable substance under conditions which appear to 
make for disintegration of the substance; and we notice in addition 
another fact, namely, that while life lasts a continuous series 
of chemical changes, at some periods less active, at others 
more, but never entirely ceasing, are always present. Now in 
this perpetual chemical change some energy is wasted, and passes 
off into the environment in the form of heat, motion, ete. How 
does the organism get sufficient extra energy, not merely to maintain 
but even to frequently increase its complex and unstable substance ? 
The extra energy might obviously be obtained if the organism con- 
tinually assimilated more complex and unstable food than the ultimate 
products into which this disintegrated protoplasm broke down. In 
confirmation of this position it is noteworthy that plant tissues which 
have reached a much lower point of evolution than animal, and whose 
tissue change is less active, require less complex food than animals. 
For synthesis energy is required, and this could be obtained as above 
from the food material; in addition it would be necessary to have a 
very slightly conducting substance, such as we have in protoplasm, to 
prevent energy from being too rapidly dissipated, while every chemical 
reaction must be extremely rarefied, as any marked evolution of energy 
would obviously lead to the destruction of the whole organism. The 
essentials for the physical aspect of protoplasmic life would therefore 
appear to be, a certain small but constant amount of surplus energy 
which leads to a very gradual substitution of the less complex into the 
more complex, and then the gradual breaking down of the more com- 
plex protoplasm thus formed, by equally gradual stages, into simpler 
products than those which had been utilised as food. 
It seems, therefore, conceivable, supposing chemical and physical 
conditions to be favourable, that a purely chemical product might be 
found which would, if situated in a suitable medium, manifest 
synthetical and analytical changes without any additional force being 
required. As further movements somewhat analogous in character to 
the amoeboid have been shown to be obtainable by chemical and 
physical conditions alone, as in the experiments of Quincke, Biitschli, 
and others, and also the various phenomena associated with chemio- 
taxis, phagocytosis, ete., appear to lead to the same conclusions, it 
would seem that the earliest forms of life might be accounted for on 
an entirely physical basis. 
In many forms of bacteria, almost all the above conditions are 
complied with; they do not include any special phenomena of move- 
ment, or show any marked reaction to stimuli. There is usually a 
special temperature at which they grow most perfectly, while below 
and above this their growth and metabolism tend to cease, and they 
