140 COSMIC PHILOSOPEY. \vr. n. 



If it be objected to this view that it obliges us to assume 

 a vast amount of differentiation and integration in the brain, 

 during the lifetime of single individuals, it may be replied 

 that the assumption is fully sustained, both by sound deduc- 

 tion and by observation. Not only does the brain increase in 

 size and heterogeneity during the first twenty-five years of 

 life, but ordinarily it increases in heterogeneity, and often in 

 size, for many years later ; and in some cases it increases in 

 heterogeneity until the end of life. The brain of a young 

 child is in homogeneity like the brain of an ape ; the furrows 

 are shallow, symmetrical, and few in number. With advanc- 

 ing years they increase in number, depth, and irregularity ; 

 and the increase is most marked in those persons who do 

 the most brain-work. In the brains of five very eminent 

 men examined by Wagner, the heterogeneity of surface 

 is described as quite astonishing. Such facts prove that 

 the operations of tliought work strongly-marked structural 

 changes in individual brains, in the course of a few years. 

 And as these strongly-marked changes are but the summing- 

 up of countless little changes in the arrangements of cells 

 and fibres, the inference is inevitable that such little changes 

 must be going on all the time. This is the testimony of 

 obseivation, and deduction might have taught us to expect 

 as much ; since the molecules of nerve-tissue are chemically 

 by far the most unstable molecules known to science, ever 

 ready to undergo metamorphosis and arrange themselves in 

 new groups. Waste and repair go on more rapidly in the 

 brain than in any other part of the body ; the cerebrum, 

 weighing between three and four pounds, receives at each 

 pulsation one-fifth of all the blood sent from the heart, and 

 if the supply is stopped for an instant, consciousness ceases. 

 Where nutritive change is so excessively rapid, such structural 

 changes as are involved in the continual setcing-up of new 

 transit-lines, must be readily effected. And quite in harmony 

 with this course of inference is the fact that, when cerebral 



