190 THE RIDDLE OF THE UNIVERSE. 



and the slower progress of the second decade, until 

 it reaches complete maturity of mind, are intimately 

 connected with a great advancement in the growth 

 and development of consciousness and of its organ, 

 the brain. But even when the pupil has got his 

 " certificate of maturity " his consciousness is still 

 far from mature ; it is then that his " world-conscious- 

 ness " first begins to develop, in his manifold relations 

 with the outer world. Then, in the third decade, we 

 have the full maturity of rational thought and con- 

 sciousness, which, in cases of normal development, 

 yield their ripe fruits during the next three decades. 

 The slow, gradual degeneration of the higher mental 

 powers, which characterises senility, usually sets in 

 at the commencement of the seventh decade — some- 

 times earlier, sometimes later. Memory, receptive- 

 ness, and interest in particular objects gradually 

 decay ; though productivity, mature consciousness, 

 and philosophic interest in general truths often 

 remain for many years longer. 



The individual development of consciousness of 

 earlier youth proves the universal validity of the 

 biogenetic law ; and, indeed, it is still recognisable in 

 many ways during the later years. In any case, the 

 ontogenesis of consciousness makes it perfectly clear 

 that it is not an " immaterial entity," but a physio- 

 logical function of the brain, and that it is, con- 

 sequently, no exception to the general law of substance. 



From the fact that consciousness, like all other 

 psychic functions, is dependent on the normal 

 development of certain organs, and that it gradually 

 unfolds in the child in proportion to the development 

 of those organs, we may already conclude that it has 

 arisen in the animal kingdom by a gradual historical 



