INBREEDING EXPERIMENTS 111 



ably with the original stock; others degenerated so rap- 

 idly that they soon became extinct in spite of every etl'ort 

 to prevent such a catastrophe. Among the families still 

 in existence, there is even evidence that vigor as a general 

 term may be divided into various causative factors and 

 that these factors may be combined in various ways, liy 

 grading each family for various characters connected 

 with vigor of growth and reproduction and then classi- 

 fying each family in numerical order for each separate 

 character, Wright has been able to show conclusively 

 that there ar^ many hereditary factors which alfect fer- 

 tility, growth and vitality and that almost any combina- 

 tion of these characters may become fixed in a family 

 through inbreeding. 



A little later we shall have occasion to speak of sev- 

 eral noteworthy end results obtained by inbreeding the 

 larger domestic mammals, but no further discussion 

 seems advisable in this place because of the lack of quan- 

 titative data. A similar statement holds for birds. 



The fruit fly, Drosopliila ynelanogaster^ is the only 

 insect which has been used for extended experiments on 

 effects of inbreeding, although there are numerous 

 examples on record where an importation of a rela- 

 tively small number of individuals has resulted in an 

 overwhelming increase — witness the gypsy moth in 

 New England. 



Castle and his co-workers ^^ bred Drosophila for 

 many generations by continuous brother and sister mat- 

 ings. After fifty-nine generations of this close inbreed- 

 ing the fertility did not appear to be reduced below that 

 shown by the original stock, although it was increased 

 by crosses between certain inbred lines. There was some 



