206 DEVELOPMENT AND HEREDITY. 



association, a reaction of the co-ordinations causing 

 development of the tail. To eventually destroy this 

 association might require a more profound disturb- 

 ance of the nervous organisation than the mere 

 cutting off of the tail after it has developed. 



A more profound disturbance is caused when 

 the nervous system itself is directly affected. The 

 operation of such a disturbance may in one or two 

 generations produce an effect in changing the 

 hereditary impulse. Thus general intemperance 

 and drunkenness in one or two, or a few genera- 

 tions, may produce insanity or other mental weak- 

 ness, which is handed on and reappears again and 

 again in future generations. This same principle 

 is illustrated in the celebrated experiments upon 

 guinea-pigs by Professor Brown-S6quard. By operat- 

 ing directly upon the nervous system, he was able 

 to cause certain well-marked changes of growth 

 to appear in the succeeding generation. 



The strongly hereditary nature of insanity and 

 other mental diseases, — they being of all diseases 

 the most certain of inheritance, — and also the 

 frequency of their appearance in combination with 

 other deformities of growth, both of these facts 

 emphasise the relation of the nervous organisa- 

 tion to the phenomena of development and 

 heredity. 



