THE MECHANISM OF HEREDITY 79 



cal factor unit with the physical unit factor of the germ 

 cells. Occasional changes in the constitution of these 

 factors, changes which may have great or small effects on 

 the characters of the organism, do occur ; but their fre- 

 quency is not such as to make necessary any change in 

 our theory of the factor as a permanent entity. In this 

 conception biology is on a par with chemistry, for the 

 practical usefulness of the conception of stability in the 

 atom is not affected by the knowledge that the atoms of 

 at least one element, radium, are breaking down rapidly 

 enough to make measurement of the process possible. 



