SPONTANEOUS GENERATION AND EVOLUTION : CONCLUSION 381 



not onlj- be caused to vaiy bj' the external influences to which they 

 are subjected, but can be guided by these along particular paths of 

 variation, so that among the variations which crop up some are 

 better adapted to the conditions than others, and these thrive better, 

 and thus alone form the basis of further evolution. In this way 

 definite tendencies of evolution are produced, which do not, move 

 blindlj' and rigidly onwards like a locomotive which is bound once 

 for all to the i-ailroad, but rather in exact response to the external 

 conditions, like an untrammelled pedestrian who makes his waj-, over 

 hill and dale, wherever it suits him best. 



The ultimate forces operative in bringing about this manj'-sided 

 evolution are the known — and although we do not recognize it as yet, 

 perhaps the unknown — ehemico-physical forces which certainly work 

 only according to laws: and that they are able to accomplish such 

 marvellous results is due to the fact that thej- are associated in 

 peculiar and often very complex different kinds of combinations, 

 and thus conform to the same sort of regulated arrangements as 

 those which condition the operations of anj- machine made by man. 

 All complex effects depend upon a co-operation of forces. This is 

 seen, to begin with, in the chemical combinations whose characters 

 depend entirely upon the number and arrangement of the elementary 

 substances of which they consist ; the atoms of carbon, hydrogen, and 

 oxygen, which compose sugar, can also combine to form carbonic acid 

 gas and water, or alcohol and carbonic acid gas ; and the same thing 

 is true if we ascend from the most complex but still inanimate organic 

 molecules to those chemical conibinations which, in a stiU higher 

 form, condition the phenomena of life, to the lowest living units, 

 the biophors. Xot only do these last differ in having life, but 

 they themselves may appear in numerous combinations, and can 

 combine among themselves to form higher units, whose characters 

 and effectiveness wiU depend upon these combinations. Just as 

 man may adjust various metallic structures, such as wheels, plates, 

 cylinders, and mainsprings in the combination which we call a watch, 

 and which measures time for us, so the biophors of different kinds 

 in the living body may form combinations of a second, third, &:c., 

 decree, which perform the different functions essential to life, and 

 by virtue of their specific, definite combination of elementary forces. 



But if it be asked, what replaces human intelligence in these 

 purposeful combinations of primary forces, we can only answer 

 that there is here a self-regulation depending upon the characters 

 of the primary vital parts, and this means that these last are caused 

 to vary by external influences and are selected by external influences, 



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