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 1 into the environment in the form of heat, motion, etc. 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, etc, 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 



