48 LECTURES ON EVOLUTION m 



of Nature is constant, and that the chain of natural 

 causation is never broken. 



In fact, no belief which we entertain has so com- 

 plete a logical basis as that to which I have just 

 referred. It tacitly underlies every process of 

 reasoning ; it is the foundation of every act of the 

 will. It is based upon the broadest induction, 

 and it is verified by the most constant, regular, 

 and universal of deductive processes. But we 

 must recollect that any human belief, however 

 broad its basis, however defensible it may seem, is, 

 after all, only a probable belief, and that our 

 widest and safest generalisations are simply state- 

 ments of the highest degree of probability. 

 Though we are quite clear about the constancy of 

 the order of Nature, at the present time, and in 

 the present state of things, it by no means 

 necessarily follows that we are justified in expanding 

 this generalisation into the infinite past, and in 

 denying, absolutely, that there may have been a 

 time when Nature did not follow a fixed order, 

 when the relations of cause and effect were not 

 definite, and w T hen extra-natural agencies interfered 

 with the general course of Nature. Cautious men 

 will allow that a universe so different from that 

 which we know may have existed ; just as a very 

 candid thinker may admit that a world in which 

 two and two do not make four, and in which two 

 straight lines do inclose a space, may exist. But 

 the same caution which forces the admission of 



