IDIOSYNCRASY. 395 



was actually differentiated (say out of pre-existing neuroglia, though 

 that is a hypothetical matter of detail) in the very act of performing 

 the trick in question. There will be no doubt whether the new fibers 

 are related to the arithmetical faculty or to the Sanskrit verbs, because 

 they were actually rendered possible as nervous tracks in the act of 

 learning decimal fractions. It is true, we may admit to the utmost 

 the intense complexity of the existing brain, and the vast number of 

 its elements involved in even the simplest muscular adjustment or the 

 simplest visual perception. Nobody feels the necessity for admitting 

 such complexity more fully than myself. One may allow with M. 

 Ribot that every act of thought must be conceived rather as a vast 

 dynamical tremor, affecting a wide plexus of very diverse nerve-ele- 

 ments, than as a single function in a single cell or fiber. One may 

 acknowledge that what one ought really to picture to one's self (at the 

 present stage of human evolution) is not so much the genesis of a new 

 cell for governing the little finger, or of a new fiber for understand- 

 ing a fact in decimal fractions, as the habituating an immense series of 

 cells and fibers, perhaps in various parts of the brain, to thrill together 

 in unison on the occurrence of a single cue. But let us thus purify 

 and dematerialize our conception as far as we like, we must neverthe- 

 less come back at last to the fact that every gain implies a modifica- 

 tion in structure, and that this modification in structure, if it is to 

 have any functional meaning and value whatsoever, must be function- 

 ally brought about. 



That such functional modifications are forever taking place in all 

 of us is a matter of common observation, as evidenced by psychological 

 facts. We are always seeing something which adds to our total stock 

 of memories ; we are always learning and doing something new. The 

 vast majority of these experiences are similar in kind to those already 

 passed through by our ancestors ; they add nothing to the inheritance 

 of the race. To use a familiar phrase in a slightly new and narrower 

 sense, they do not help to build up " forms of thought " ; though they 

 leave physical traces on the individual, they do not so far affect the 

 underlying organization of the brain as to make the development of 

 after-brains somewhat different from previous ones. But there are 

 certain functional activities which do tend so to alter the development 

 of after-brains ; certain novel or sustained activities which apparently 

 result in the production of new correlated brain-elements or brain- 

 connections, hereditarily transmissible as increased potentialities of 

 similar activity in the offspring. If this is not so, then there is no 

 meaning at all in the facts collected by Mr. Galton, or, indeed, for the 

 matter of that, in the common facts of human experience as to heredi- 

 tary transmission of faculties for acquired pursuits of any sort. If 

 the children of acrobats make the best tumblers, if the descendants 

 of musical families make the best singers and composers, if a great 

 thinker or a great painter is usually produced by the convergence of 



