226 CHILDHOOD OF ANIMALS 



of a macaque is still smaller, but larger than that of a cebus 

 monkey. It is better not to include lemurs in this series. 

 Although they are more closely related with apes and monkeys than 

 with any other living animals, they form an independent series, the 

 lowest members of which have smooth and small brains, yet the 

 higher members of which have brains which surpass in develop- 

 ment those of the lowest monkeys. 



But there is a more important consideration even than size. 

 The cerebrum, the great mass of the brain that fills most of the 

 hollow of the skull, is smooth in the lowest forms save for a few faint 

 wrinkles, which become more conspicuous and more numerous 

 successively in the cebus, macaque and chimpanzee, until they 

 attain the high complexity shown in the human brain. The corre- 

 spondence is so close that we may say almost definitely that, at least 

 inside the great groups of mammals, the length of the period of 

 youth increases with the size and complexity of the brain. 



Without going too deeply into anatomical details, we can take 

 the comparison a little further. If a slice cut through a cerebrum 

 be examined even with the naked eye, it will be seen to consist of 

 a greyish layer, closely following the external contour, and a deeper- 

 seated white mass. If you compare two stretches of coast occupying 

 roughly the same space on a map, as, for instance, the east coast 

 of Wicklow and Wexford with the south-west coasts of Kerry and 

 Cork, you will see at once that the coast-lines corresponding with 

 the same inland area may differ enormously in length, according 

 to whether they present an even front to the sea, or are twisted 

 into bays and fiords. So also in brains of nearly the same size, 

 the amount of grey matter is much greater in one that is folded 

 and convoluted than in one that is nearly smooth. The duration 

 of youth in animals belonging to the same group varies with the 

 relative amounts of grey matter contained in their brains. 



I am not going to discuss here whether the brain be an organ of 

 the mind, played on by some immaterial entity, as a musician plays 

 on a musical instrument, or whether mental qualities be emanations 

 of the brain as bile is a secretion of the liver. It is enough that 

 the mental powers are definitely associated with the grey matter, 

 and that their development and education write a record upon it. 

 The grey matter contains the nerve-cells of the brain. A fully 

 developed nerve-cell may be compared with a spider seated in the 

 centre of a web which is an actual set of outgrowths from itself, 

 .Some of the fibres of the web are in connection with nerves ; indeed, 



