25 2 HEREDITY AND DISEASE 



cases, but it is probable, as Virchow has maintained, that some 

 new beginnings which are now — looking backward— regarded 

 as normal steps in progressive evolution would at the outset 

 have been claimed by the pathologist as hints of fresh disease. 

 Leaving microbic and acquired diseases out of account, we 

 may safely say that various processes of hypertrophy and 

 atrophy which are associated with disease in a well-finished 

 organism like man are, as it were, recrudescences of important 

 steps in past evolution. The persistence of germinal activity 

 in a patch of cells may give rise to a tumour, but is it not, as 

 it were, an echo of the power that lower animals have of regener- 

 ating lost parts ? So it may be that some of the cerebral varia- 

 tions which we call for convenience "nervous diseases" are 

 attempts at progress. 



Diseases due to Innate Predispositions and to Acquired 

 Modifications. — From the biologist's point of view diseases are 

 of two sorts : (i) they are abnormal or deranged processes, 

 which have their roots in germinal peculiarities or defects 

 (variations, to start with), which express themselves in the 

 body to a greater or less degree according to the conditions 

 of nurture ; or (2) they are abnormal or deranged processes 

 which have been directly induced in the body by acquired 

 modifications — i.e. as the results of unnatural surroundings or 

 habits, including the intrusion of parasites. Often, moreover, 

 an inborn predisposition to some deranged function may be 

 exaggerated by extrinsic stimuli, as in the case of gout,* or when 

 a phthisical tendency is aggravated by the intrusion and 

 multiplication of the tubercle bacillus. That is to say, deranged 

 processes which are primarily due to germinal variation often 

 afford opportunity for equally serious disturbances which 

 must be referred to exogenous modifications. A rheumatic 

 tendency may be fatally aggravated by inappropriate nutrition. 



* It is now suggested, however, that gout is due to the toxic effect of 

 some germ or germs. 



