THE EVOLUTION OF MIND 



of transit lines which cannot be obliterated. No 

 effort can prevent their occasional recurrence 

 along these lines, or establish a new plexus of 

 transit lines, involving the derangement of the 

 old ones. Late in life, when the ratio of repair 

 to waste is greatly diminished, when the nutri- 

 tion of the cerebral tissue is impaired, when the 

 pulling to pieces and putting together of molec- 

 ular clusters in which nutrition consists goes on 

 slowly, then the formation of new sets of tran- 

 sit lines becomes especially difficult ; and hence, 

 as we say, the shaking off of old habits and 

 prejudices, and the acquiring of new and strange 

 ideas, is next to impossible. It is proverbially 

 hard to teach an old*dog new tricks. We may 

 here also see why it is impossible to learn or 

 to carry on complicated thinking when in a state 

 of anaemia : the nutritive changes go on too 

 slowly. Changes in memory further illustrate 

 the theory. In youth, when the excess of repair 

 over waste is at the maximum, but few discharges 

 through any transit fibre are needful in order 

 to work a permanent nutritive change, setting 

 up a line of communication which shall last 

 through life : hence learning is easy and rapid, 

 and memory is powerful. In old age, when 

 waste is slightly in excess of repair, and both 

 are at the mimimum, a great many discharges 

 are necessary for the achievement of any per- 

 manent nutritive change, — hence learning is 

 217 



