104 MODES OF RESEARCH IN GENETICS 



point one is confronted with the necessity for a 

 definition of inbreeding which shall be at once 

 precise and general, that is, such as to include all 

 of the many diverse ways in which this sort of 

 breeding may be practiced. A great number of 

 definitions of the concept "inbreeding" have been 

 proposed in the literature of genetics. I shall not 

 attempt to review these definitions here, since to 

 do so would serve no useful purpose in the present 

 connection. A careful consideration of them is 

 bound, I think, to lead one to the conclusion that 

 they have been, in general, based on grounds of 

 practical expediency rather than critical biological 

 analysis. 



Clearness and simplicity of thinking will be 

 gained by approaching the problem de novo. 

 Leaving aside for the moment all consideration of 

 details as to how a particular piece of inbreeding 

 is done, it is clear that underlying all definitions of 

 inbreeding is to be found the concept of a narrowing 

 of the network of descent as a result of mating 

 together at some point in the network individuals 

 genetically related to one another in some degree. 

 Let us take this as our basic concept of inbreeding. 

 It means that the number of potentially different 

 germ-to-germ lines or "blood-lines" concentrated 

 in a given individual animal is fewer if the 

 individual is inbred than if it is not. In other 

 words, the inbred individual possesses fewer differ- 

 ent ancestors in some particular generation or gen- 



