282 THE PRINCIPLES OF HEREDITY 



of nervousness, myxcedema of lethargy. Brain disease may 

 lead to positive lunacy and mania. An early union of the 

 cranial bones is a constant source of idiotcy ; the growth of 

 the brain and mind being checked, the microcephalic idiot, 

 unable to make those acquirements by means of which 

 rational people balance and direct their instincts, behaves 

 like a lower animal. He follows his instincts. Physical 

 strength and activity predispose to arrogance and courage ; 

 physical weakness to timidity and meekness. 



451. But apart from the mental acquirements due to 

 bodily conditions are those due to the world outside the 

 individual. To take an illustration. Imagine twin infants in 

 a cot, one awake and the other asleep ; suppose an event 

 happens that alarms the waking child, but leaves the other 

 unaffected ; suppose again that subsequently another event, 

 observed by both children, occurs, which, owing to the 

 apprehension and nervous irritability engendered by the 

 previous event, again alarms the first child and thus increases 

 its irritability, but, because of the previously undisturbed 

 equanimity, again leaves the second unaffected by fear; 

 imagine this process repeated; then, though the original 

 cause of fear were quite forgotten, the one child might well 

 grow up of a much more timid and nervous disposition than 

 the other. In which case every one would speak of the 

 former as naturally (i. e. innately, instinctively) more timid 

 than his brother, though in fact his timidity would be 

 acquired. So might many peculiarities of mental character 

 arise. I have taken an hypothetical case ; but most of us 

 can remember experiences, unimportant in themselves and 

 occurring later in life, when the individual is much less im- 

 pressionable, which have been so accentuated and magnified 

 by subsequent occurrences as to change the whole character, 

 and with it the career, of some person. 1 



452. To take another illustration. Identical twins are 

 extremely alike physically. Presumably, since mental 

 characters are correlated to cerebral structures, they are as 

 much alike as regards innate mental traits instincts and 

 capabilities. Suppose such a pair were separated in infancy, 

 one being placed in a ploughman's hut, the other being given 

 a career in a good school and university. Presently they 

 would differ mentally in toto ccelo. Physically they would 

 develop much alike. The ploughboy's hands would be a 

 little rougher and his skin a little more tanned. The main 



1 In my own case five minutes' careless conversation when a boy led 

 to a gradual but profound change in my entire character and career. 



