382 THE EVOLUTION TPIEORY 



that is, are chosen for survival or excluded from it Thus combina- 

 tions of living units must always result which are appropriate to 

 the situation at the moment, for no others can survive, although, 

 as we have seen, they must arise. This is our view of the. causes 

 of the evolution of the world of organisms ; the living substance 

 may be compared to a plastic mass which is poured out over a wide 

 plain, and in its ceaseless flowing adapts itself to every unevenness, 

 flows into every hole, covers every stone or post, leaving an exact 

 model of it, and all this simply by virtue of its constitution, which 

 is at first fluid and then becomes solid, and of the form of the surface 

 over which it flows. 



But it is not merely the surface in our analogy which determines 

 the form of the organic world ; we must take account not only of the 

 external conditions of existence, but also of the constitution of the flow- 

 ing mass, the living substance itself, at every stage of its evolution. 

 The combination of living units which forms the organism is different 

 at each stage, and it is upon this that its further evolution depends ; 

 this difference determines what its further evolution may be, liut 

 the conditions of life determine what it mud be in a particular case. 

 Thus, in a certain sense, it was with the first biophors, originating 

 through spontaneous generation, that the whole of the organic world 

 was determined, for their origin involved not only the physical con- 

 stitution by which the variations of the organism were limited, but also 

 the external conditions, with their changes up till now, to which 

 organisms had to adapt themselves. There can be no doubt that 

 on another planet with other conditions of life other orgariisms would 

 have arisen, and would have succeeded each other in diverse series. 

 On the planet Mars, for instance, with its entirely different conditions as 

 regards the proportions in weight and volume of the chemical elements 

 and their combinations, living substance, if it could arise at all, would 

 occur in a different chemical composition, and thus V^e equippc^d with 

 different characters, and without doubt also with quite different 

 possibilities of further development and transformation. The highly 

 evolved world of organisms which we may suppose to exist upon Mars, 

 chiefly on the ground of the presence of the i-emarkable straight 

 canals discovered by Schiaparelli, must therefore be thought of as 

 very different from the terrestrial living world. 



But upon the earth things could not have been very ib' He rent 

 from what they actually are, even if we allow a good deal to cliance 

 and assume that the form of seas and continents might have been 

 quite different, the folding of the surface into mountains and valleys, 

 and the formation of rents and fissures, with the volcanoes that burst 



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