EFFECTS OF INBREEDING AND CROSSBREEDING. 



29 



SEX RATIO. 



Since the experiment began, there have been no changes in sex 

 ratio which can be relied upon. The number of males per 100 females 

 in different years is given in Table 22. The results are presented 

 graphically in Figure 11. The most interesting point brought out by 

 the figures is the small size of the fluctuations and the absence of any 

 parallelism with those of the other characters. Evidently the favor- 

 able conditions of 1910 and 1914 and the unfavorable conditions of 

 1911, 1916, and 1917 are, like inbreeding itself, without effect on 

 the sex ratio. 



1906 



/30 



/20 



/JO 



JOO 



J906 J9J0 



19/2 



/9/4 



19/6 



/9/6 J920 



;B 



i 



* i 



/ • • 



' \ i 



——, , ^ — i — 



»• i /\ • 



i x \ ' 



>» i /\. / \ 



S\ • / x #» / \ 



x -L \£ ^» i \ 



•y / 



» / 



\—? 



90 



60 



FlG. 11.— Sex ratio. Number of males per 100 females. Inbreds (A) and controls (B), 1906 to 1920. 



TESTS FOR DISEASE RESISTANCE. 



Tests were made in 1911 on the resistance of inbred and normal 

 guinea pigs to tuberculosis. The methods used and their results 

 are described, as follows, in a report by Dr. E. H. Riley: 



The relative susceptibility to disease of inbred and normally bred guinea pigs was 

 tested by inoculating inbred animals as principals and normally bred animals as 

 checks with an equal quantity of material containing tubercle bacilli. This work 

 was done in cooperation with Dr. E. C. Schroeder, Superintendent of the Bureau of 

 Animal Industry Experiment Station, Bethesda, Md. The inoculations were 

 made under his direction, and facilities were supplied by him for the housing and 

 care of the animals during the progress of the disease. The results of the autopsies, 

 which were made at the Experiment Station, for eight different tests are recorded in 

 Table 5. 



The principals used in the first two tests were inbred, brother and sister, for six 

 generations, and were selected from the inbreeding experiment. The checks were 

 not inbred, but were selected from the general stock of guinea pigs bred and reared 

 at the experiment station. Aside from having been born and reared in different 

 buildings, they were raised under practically the same conditions of feeding, care, 



