168 THE GROWTH OF GROUPS 



disease that is dealt with in books devoted to general 

 medicine. 



Among the many kinds of insanity that are commonly 

 met with, paralytic dementia is peculiar in two respects. 

 It is definite in course and symptoms and it is produced 

 by the invasion of a parasite. The disease cannot occur 

 in the absence of the specific organism. If our know- 

 ledge of the causation of insanity in general were a 

 mass of conjectures containing a single fact, this would 

 be that fact. There are of course other influences in the 

 causation of this disease, since everybody infected with 

 the parasite does not become paralytic, but that does 

 not lessen the certainty that the essential cause of para- 

 lytic dementia is the specific organism. It is hardly less 

 certain that the general physician feels on sure ground 

 as regards diagnosis and prognosis when he meets a case 

 of paralytic dementia, but not so when confronted with 

 other kinds of mental disorder. All those others are for 

 the alienist to deal with, and sometimes one notices a 

 certain scepticism in the mind of the general physician 

 as to whether the various types described by the alienists 

 are definite enough to be clearly recognizable. But there 

 is of course no justifiable reason for doubting the reality 

 of the types. 



This distinction between paralytic dementia and the 

 others is probably due to the fact that the former is caused 

 by an invading parasite while the latter are not so caused. 

 How, then, are they caused ? asks the alienist, who finds 

 himself face to face with exactly the same problem that 

 confronts the biologist when seeking to know the cause 

 of variation. Let us all admit that the cause is unknown, 

 but let us recognize that the biologist and the alienist are 



