222 READINGS IN BIOLOGICAL SCIENCE 



difficulty in this interpretation, however, in that not all the offspring of 

 matings with both parents diseased are afflicted. This is susceptible of 

 alternative explanations. It may be that the inheritance is dependent on 

 multiple factors in which case the line between dominant and recessive is 

 not necessarily clean cut. Some characters may be dominant, others re- 

 cessive and the actual behavior of the individual is the result of a kind of 

 balance. 



Another possible explanation is that the disease itself is not inherited but 

 only the liability to contract it. That is to say, an individual potentially sen- 

 sitive by reason of inheritance may escape the influence of the environmen- 

 tal factor and never reveal his latent tendencies. 



DISORDERS AND DEFECTS OF THE CENTRAL 

 NERVOUSSYSTEM 



Popular interest in inheritance, alike of normal and abnormal qualities, 

 naturally reaches its highest when the nervous system is considered. From 

 the medical point of view we are here dealing with the diseases referable 

 to a single organ. Gross defects of development occur and are likely to 

 be lethal before their general effects on function can become manifest. 

 Finer defects in structure may well be common but may escape recogni- 

 tion. The brain and spinal cord are affected in the course of infectious dis- 

 eases which are to be considered as general infections, and also are the 

 seat of infections primary in or affecting, chiefly themselves, e. g. polio- 

 myelitis, encephalitis lethargica, and cerebrospinal meningitis. With ref- 

 erence to these what has been said with regard to the inheritance of im- 

 munity or susceptibility to infectious disease generally doubtless has some 

 application in principle but we have no specific knowledge of inherited 

 influences in the particular cases. The functional disorders of the nervous 

 system are manifest in almost infinite variety and the study of them has 

 gradually become a very intricate specialty. From our present point of 

 view only certain outstanding selections can be considered for purposes 

 of illustration. 



Feeblemindedness has a peculiar interest. The condition (one, it may 

 be supposed, of limited development) rests in some instances on an in- 

 herited basis as made evident by careful and competent scientific investi- 

 gation. Is feeblemindedness a disease? Obviously it may be so regarded in 

 the social sense, since on a purely practical basis a highly developed society 

 is forced to maintain large institutions for the care of such of its offspring 

 as are unable to maintain the pace. From the pathological standpoint it 

 is hardly to be looked upon as a disease except in the most extreme or 

 particular instances. But when one begins to discriminate on a quantita- 

 tive basis all the standards must be arbitrarily chosen. The question clearly 

 becomes an academic one when purely practical standards are disregarded. 



