492 HUMAN BIOLOGY 



respectively, but are not so regarded under the more rigid 

 definition of inheritance. 



Even the circumscribed definition of inheritance as here 

 given may not be wholly accurate. There is much reason to 

 beheve that injury to the parents by long-continued exposure 

 to certain poisons such as alcohol or lead may affect the 

 offspring unfavorably and it is also probable on the basis of 

 animal experiments that exposure of the parents to roentgen 

 rays may, under certain conditions, result in altered if not 

 abnormal descendants. In so far as these influences may be 

 manifest through action on the male parent it can only 

 be by some affection of the germ cell itself and it would 

 probably be impossible to frame an entirely adequate defi- 

 nition of inheritance in which these preconceptual influences 

 are justly accounted for. These may for purposes of definition 

 be recognized and passed over. 



The outstanding achievement of genetic study has been to 

 show that as a broad biological principal the most diverse 

 general characters can be analyzed into an infinity, almost, 

 of combinations of less inclusive specific unit characters 

 which are inherited independently in principal. Actually 

 they are inherited either separately or in small and apparently 

 "chance constituted" linkage groups. There is every reason 

 to suppose that the mechanism of human inheritance com- 

 pletely conforms to this "Mendelian" scheme. That it 

 does so has been demonstrated for a considerable number 

 of characteristics. 



"Disease" is a general concept sufficiently defined for many 

 purposes as any condition of body or mind which departs 

 from "perfect health." A precise definition which shall be 

 more critical than this and cover all the manifestations of 

 morbid processes is extremely difficult to formulate. It 

 would greatly simplify this, and many other discussions of a 

 similar nature, if an all-inclusive definition could be framed, 

 but the attempt would be hopeless and misleading in the 

 nature of the facts. It is well to recognize this clearly at 

 this point because there is a very general assumption or 

 belief that people are quite definitely divided into two 

 classes, those who are born healthy and of sound constitu- 

 tion, and those who come into the world otherwise. All 



