104 HEREDITY 



gained any considerable knowledge of the causation 

 and essential nature of the vast majority of all 

 diseased conditions. The study of the inheritance 

 of disease entered on its first fruitful stasfe when 

 Pasteur and his followers taught us that nearly 

 all disease is due to the invasion of the body by 

 microbes or bacteria. As it is daily becoming more 

 difficult to name any important diseases — save prob- 

 ably cancer — that are not of microbic origin, we shall 

 here take for granted a practical identity between 

 " disease " and " disease of bacterial causation." 



Plainly the greater part of our discussion will have 

 little bearing at this stage on the general problems 

 of heredity, but will be concerned rather with the 

 habits of various bacteria, and the possibility of 

 their communication to the germ or the embryo. 

 Let us take, for instance, the most important of all 

 diseases, which probably slays about one in five, six, 

 or seven of all the human beings that die upon the 

 earth. We now know that "consumption" and a 

 hundred other diseased conditions are due to the 

 invasion of some part of the body by a minute plant 

 called the tubercle bacillus or hacillws tuberculosis. 

 It is still widely believed that tuberculosis is 

 hereditary. 



But to the modern pathologist this statement can 

 only mean that the tubercle bacillus may invade the 

 germ-cells by direct passage from an infected parent ; 

 or may enter the embryo from the body of an 

 infected mother. Thus the child at birth will contain 

 tubercle bacilli. This the pathologist would term 

 congenital or hereditary tuberculosis. Plainly, how- 

 ever, it would be desirable to use these terms 



