24 HEREDITY IN RELATION TO EUGENICS 



Unfortunately, such a consummation cannot for some time 

 be achieved. The reason for the delay is twofold. First, 

 we do not yet know all of the unit characters in man; second, 

 we can hardly know in advance which of them are due to 

 positive determiners and which to the absence of ^ch. 



Unit characters can rarely be recognized by inspection. 

 For example the white coat color of a horse is apparently 

 a simple character, but experimental breeding shows that 

 it is really due to several independently inheritable factors. 

 The popular classification of traits is often crude, lagging 

 far behind scientific knowledge. Thus insanity is frequently 

 referred to a single trait. It is clear, however, that insanity 

 is a result merely and not a specific trait. Some cases of 

 insanity indicate an innate weakness of the nervous system 

 such as leads it to break down under the incidence of heavy 

 stress; other cases of insanity are due to a destruction of 

 a part of the brain by a wound as, for instance, of a bullet. 

 In some cas^s, through infection a wide-spread deteriora- 

 tion of the brain occurs; in other cases a clot in a cerebral 

 blood vessel may occlude it, cut off nutrition from a single 

 locality of the brain and interfere with movements that 

 have their centres at the affected point. Now these four 

 results cannot be said to be due to the same unit defect; 

 and they can hardly be compared in the study of heredity. 



On the other hand, the original expectation that progress 

 must wait on a complete analysis of unit characters proves 

 not to be correct. There are a number of forms of insanity 

 that are sharply separable symptomatically and structurally 

 which have a common basis in that they are due to a nervous 

 weakness; and "nervous weakness" may behave in heredity 

 with relation to "nervous strength" like a lower grade, or 

 the absence, of a highly developed character. Even with- 

 out a complete analysis of a trait into its units we may still 

 make practically important studies by using the principle 



