EXTENSION OF EVOLUTIONARY THEORIES 349 



account. It is of their very essence that they be in 

 harmony with the external conditions of life, and these 

 are brought about by a totally different force — the force 

 that effects the geological changes of the earth. As it 

 is a fact that the organisms have always been modified 

 in correspondence with the geological changes of the 

 earth's surface, since the adaptations always harmonise 

 with the new condition of the earth, the two forces 

 must go together like two mutually regulated clocks. 

 And as the forces have nothing to do with each other, 

 we can only explain their agreement by postulating a 

 third force that controlled them from the start. If we 

 do this we abandon all attempt at scientific explanation. 



If we want to explain a phenomenon satisfactorily we 

 have to bring it into line with a more general and better 

 known phenomenon within our experience. But we 

 must not ascribe to this phenomenon any characteristics 

 which we do not really know to be present in it ; and 

 in experience we are only acquainted with the corporeal 

 and with movement, matter and force, the material of 

 chemistry and physics. 



What do we mean by reducing one phenomenon to 

 another ? It means that we can show the one to be a 

 necessary consequence of the other. The phenomenon 

 to be explained must have a cause, of which it is the 

 necessary effect, v We have, therefore, satisfactorily 

 explained a phenomenon when we show that it is the 

 effect of another of which we know by experience that 

 under certain known conditions it is bound to produce 

 the effect in question, and that these circumstances are 

 actually given. Thus we explain the luminosity of 



