EVOLUTION THE MASTER-KEY 



ably produced these canals and their arrangement, 

 and this is a result of our ancestors' converse with 

 phenomena. Function has produced structure, and 

 though we are not born with any innate idea of 

 space, yet we are endowed with these canals, 

 the products of ancestral experience, and in this 

 regard our percipient apparatus is thus very far 

 from being a tabula rasa a blank sheet of paper 

 but has within it, potentially or implicitly, so to 

 speak, not the idea of space, but the materials with 

 which that idea may be attained so soon as ex- 

 perience begins. 



This theory that each of us is indebted for his 

 mental configuration and aptitudes to the manifold 

 experiences of millions of ancestors has a direct 

 bearing on what I have called, in a previous chap- 

 ter, "the test of truth." As we have seen, a truth 

 of the highest certainty is one the negation of 

 which is inconceivable. But the validity of this 

 criterion is incalculably enhanced by the considera- 

 tion that the inconceivableness depends not merely 

 on individual experience, but is a product of in- 

 dividual experience plus the total result, "up to 

 date," of the experience of the race. It must 

 certainly be admitted, as Mill argued, that propo- 

 sitions which appeared inconceivable to one age 

 may cease to be so regarded by a later generation ; 

 but, nevertheless, there is no surer criterion at our 

 disposal, and, though it is by no means absolutely 

 sure, yet it may be accorded a much higher measure 

 of confidence, when we regard the structure of the 

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