PRACTICAL PROBLEMS 351 



544. The severity of Natural Selection with regard to the 

 insane has been greatly reduced, and, as in all similar cases, 

 characters which selection formerly eliminated are tending 

 to become more common. Here we have to deal not 

 necessarily with regression. Except in the case of congenital 

 idiots, regression probably plays but a small part in the 

 causation of insanity at any rate regression of the brain 

 and mind en masse. Partial regression, however, may account 

 for many cases of insanity, though we have no means ot 

 ascertaining it. The huge brain of man is a very complex 

 and delicate machine. A defect (an unfavourable variation, 

 progressive or regressive) in any of its parts is apt to throw 

 the whole out of gear; and, like other variations, such a 

 defect, such a predisposition to insanity, tends to be in- 

 herited. In this case, as in others, there may be blended, 

 particulate, or exclusive inheritance ; or parental insanity, 

 latent in the child, may reappear in succeeding generations. 

 If the insane parent departs very widely from the normal 

 type, exclusive inheritance in offspring or descendants is, 

 according to Mendelian principles, very probable. Here also 

 it follows, unless we find means to check the output of 

 children by the mentally unsound, lunatics will multiply 

 until the State is able no longer to bear the expense of their 

 maintenance. 



545. Whether the theory of heredity formulated in this 

 volume be or be not correct must be judged by the reader. 

 I have endeavoured to found its conclusions wholly on 

 evidence which may be verified, and as much as possible on 

 evidence which falls within the experience of most educated 

 men. I have tried also to demonstrate the importance of 

 the subject. However greatly I may be shown in the future 

 to have erred in this or that particular, it is evident that 

 the time has arrived when the subject of heredity must 

 engage the attention of thinking people, especially medical 

 men. A knowledge of it should be a part of the ordinary 

 professional equipment of the physician. Apart from its 

 intellectual and educational value, the subject possesses a 

 practical interest. We cannot expect botanists and zoolo- 

 gists, whose studies are singularly exclusive, to pay much 

 attention to the recent past or immediate future of Man. 

 Historians, too, are preoccupied with minor political changes. 

 A great field of knowledge, in which biology and political 

 history meet, remains untilled. It is the task and the duty 

 of medical men to gather its harvests and apply them to the 



