CHAPTER XII 



INBREEDING 



FOR species with separate sexes the term "inbreeding" is 

 used to express either the union between brothers and sisters 

 or between offspring and parent, in one or more gen- 

 erations. It would have been convenient to apply the word 

 "interbreeding" to unions between individuals of the same 

 species individuals not standing in close filial relationship 

 and to contrast this process with " cross-breeding " (or cross- 

 ing or hybridizing) where the union is between different races, 

 varieties, or species. Convenient as these distinctions might 

 be, they have not been adhered to by writers, and the term 

 " interbreeding " is sometimes used where inbreeding seems the 

 preferable term. 1 



It is a general belief amongst breeders that inbreeding leads to 

 injurious results. If this statement is intended to apply to all 

 living species of animals and plants it is probably not true, but 

 that it maybe true in certain cases seems fairly well authenticated. 

 Even in these latter cases the question may fairly be raised 

 whether the results may not be due, in part at least, to the similar- 

 ity of the external conditions under which the individuals have 

 been kept. In other words, there is some evidence to show that 

 if the external conditions are different for different individuals, 

 the injurious effects of inbreeding are lessened or disappear in 

 some cases ; and conversely the breeding of individuals of dif- 

 ferent 'descent if subjected to identical external conditions 

 might show the same deleterious effects as those associated with 



1 Darwin often used interbreeding where inbreeding in the above sense is 

 meant. 



1 86 



