2l6 READINGS IN BIOLOGICAL SCIENCE 



This distinction is of real importance to a clear understanding of the 

 relations between inheritance and disease. The biblical dictum that the 

 sins of the fathers are visited on their offspring for generations has been 

 considered in recent times to be particularly appUcable to one contagious 

 disease, syphilis. Children suffering severely from this disease are fre- 

 quently brought into the world at or before the normal birth period. It 

 is now considered a certainty that in these cases the child is infected at some 

 point in its fetal life definitely subsequent to its conception. In any event, 

 it is infected with an extraneous microorganism carried by one or both 

 parents. On the other hand, it is known, by animal experiment at least, that 

 the offspring of an immune mother are apt to show more than the usual 

 resistance to certain diseases for some time after birth. This, it is recognized, 

 is due to the transfer of protective substances in a passive way from the 

 mother either through the membranes separating the fetal from the ma- 

 ternal circulation hi utero or in the milk during the first days of life. Under 

 the older definition, these instances would be considered to be cases of 

 inherited disease or inherited immunity respectively, but are not so re- 

 garded 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 believe 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 definition of in- 

 heritance 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 principle 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 principle. 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 completely conforms to this "A4en- 

 delian" 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 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 



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