THE SIZE OF CEREBRUM IN MAMMALIA 49 



deserves. We can not suppose that the extinct Rhinocéros Titanotherium 

 was really defective in the essential control of its organisation by the céré- 

 bral nerve-centres. Probably could we see the two créatures alive side by 

 side, we should not detect any defect in the manifestations of the nervous 

 System in Titanotherium as compared with Rhinocéros; just as we do not 

 remark any such obvious inferiority when we compare a iizard and (let us 

 say) a mouse. The organism with the lesser cerebrum is in each case in 

 spite of the smaller mass of cérébral nerve-tissue, an efficient and adéquate 

 pièce of living mechanism. 



In what then does the advantage of a larger cérébral masa consist? 

 What is it that the more récent Mammalia bave gained by their larger 

 brains? Why bas there been this sélection in ail lines of animal descent 

 of increased cérébral tissue? 



I think we gain a key to the answer to this question by a considération 

 of the différences of cérébral quality between man and apes. Man is born 

 with fewer ready made tricks of the nervous centres — those performances 

 of an inherited nervous mechanism so often called by the ill-defined term 

 *' instincts " — than are the monkeys or any other animal. Correlated with 

 this absence of inherited ready-made mechanism, man bas a greater capa- 

 city for developing in the course of his individiial growth similar nervous 

 mechanisms (similar to but not identical with those of " instinct ") than 

 any other animal. He bas a greater capacity for " learning " and storing 

 his individiial expérience, so as to take the place of the more gênerai inhe- 

 rited brain-mechanisms of lower mammals. Obviously such brain-mecha- 

 nisms as the individual thus develops (habits, judgments, etc.) are of greater 

 value in the struggle for existence than are the less specially-fitted instinc- 

 tive in-born mechanisms of a race, species or genus. The power of being 

 educated — " educabilily " as we may term it — is what man possesses in 

 excess as compared with the apes. I think we are justified in forming the 

 hypothesis that it is this " educability " which is the corrélative of the 

 increased size of the cerebrum. If this hypothesis be correct — then we may 

 conclude that in ail classes of Vertebrata and even in many Invertebrata — 

 there is and bas been a continuai tendency to substitute " educability" for 

 mère inherited brain-mechanisms or instincts, and that this requires 

 increased volume of cérébral substance. A mère spoonful of cérébral tissue 

 is sufficient to carry abundant and highly efficient instinctive mechanisms 

 from génération to génération, but for the for more valuable capacity of 

 elaborating ncio brain mechanisms in the individual as the resuit of the 

 individual's expérience of surrounding conditions, a very much larger 

 volume of cérébral tissue is needed. 



Thus it seems probable that " educability " bas increased in those Mam- 

 malia which bave survived. The ancient forms with small brains though 



