548 ON HEREDITY. 



Although a sufficient number of cases has now been put on record to 

 jirove that in some families one or other kind of congenital deformity 

 may be hereditarily transmitted, yet I do not wish it to be supposed 

 that congenital malformations may not arise in individuals in whom no 

 hereditary tendency can be traced. 



The variations I have spoken of as congenital malformations arise, as 

 a rule, before the time of birth, during the early development of the 

 individual; but there is an important class of cases in which the evi- 

 dences for hereditary transmission is more or less strong, which may 

 not exhibit their peculiarities until months, or even years, after the 

 birth of the individual This class is spoken of as hereditary diseases, 

 and the structural and functional changes which they produce exercise 

 most momentous influences. Sometimes these diseases may occasion 

 changes in the tissues and organs of the body of considerable magni- 

 tude, but at other times the alteration'is much more subtle, is molecu- 

 lar in its character, requires the microscope for its determination, or is 

 even incapable of being recognized by that instrument. 



Had one been discussing the subject of hereditary disease twenty 

 years ago, the first exam[)le probably that would have been adduced 

 would have been tuberculosis, but the additions to our knowledge of 

 late years throw some doubt upon its hereditary character. There can, 

 of course, be no question that tubercular disease propagates itself in 

 numerous families from generation to generation, and that such families 

 show a special susceptibility or tendency to this disease in one or other 

 of its forms. But whilst fully admitting the pre-disposition to it which 

 exists in certain families, there is reason to think that the structural 

 disease itself is not hereditarily transmitted, but that it is directly ex- 

 cited in each individual in w horn it appears by a process of external infec- 

 tion due to the action of the tubercle bacillus. Still, if the disease itself 

 be not inherited, a particular temperament which renders the constitu- 

 tion liable to be attacked by it, is capable of hereditary transmission. 



Sir James Paget,* when writing on the subject of cancer, gives statis- 

 tics to show that about a quarter of the persons affected were aware of 

 the existence of the same disease in other members of their family, an d 

 he cites particular instances in which cancer was present in two and 

 even four generations. He had no doubt that the disease can be in- 

 herited — not, he says, that strictly speaking, cancer, or cancerous mate- 

 rial is transmitted, but a tendency to the production of those conditions 

 which will finally mdnifest themselves in a cancerous growth. The germ 

 from the cancerous parent must be so far different from the normal as 

 after a lapse of years to engender the cancerous condition. 



Heredity is also one of the most powerful factors in the production 

 of those affections which we call gout and rheumatism. Sir Dyce Duck- 

 worth, the latest systematic writer on gout, states that in those families 

 whose histories are the most complete aiul trustworthy the influence is 



* Lectures on Surgical Pathology, 3d ed., London, 1870. 



