CHAPTER XL 



Heredity (continued*)— The Fixity of Structural Characters in Inverse Propor- 

 tion to their Age, Ancestral or Individual — All Diseases of Recent Ancestral 

 Date — The Vis Medicatrix Naturae — The Tendency of Recently Acquired 

 Characters to Disappear under Ancestral Forms of Environment. 



The Fixity or Stability of Structural Peculiarities. — 



All living organisms tend to vary — that is to say, to take 

 on new characters from generation to generation. In this 

 way, structural peculiarities are continually being acquired and 

 thrown off again. This is true whether we regard many 

 generations of a successive species or a single individual. As. 

 regards the former, we find structural characters which have 

 belonged to a species during thousands of generations gradually 

 disappearing as the environment of the organism alters ; and,, 

 as regards the individual, we shall find that structural pecu- 

 liarities acquired by the individual himself may similarly be- 

 dropped, — such, for instance, as habits (which are nothing else 

 than the functional manifestations of structural acquisitions) 

 and pathological states of the tissues — namely, disease ; and 

 further, that under disease racial characters are apt to dis- 

 appear. 



Now, the fixity of such structural peculiarities, be they racial 

 or individual, depends in large measure upon their age, ances- 

 tral and individual. Those structural peculiarities which are of 

 greatest age are generally the most fixed and stable ; on the other 

 hand, those last acquired are usually the most unstable — most apt 

 to disappear. Speaking of the race as a whole, the age of a 

 structural peculiarity is measured by the number of successive 

 generations in which it has occurred : the greater this number^ 

 the more tenacious is the structural character in question ;. 

 while as regards characters acquired by the individual himself, 

 those are the most fixed which he acquired earliest. Concerning; 



