110 INBREEDING AND OUTBREEDING 



fertile and longer lived than many strains of stock rats. 

 It is clear that this was the result of Mendelian recom- 

 bination for the two lines A and B were in the end some- 

 what different. The rats of line A were slightly more 

 fertile, attained sexual maturity earlier, and lived longer 

 than those of line B. If this evidence were not sufficient, it 

 is supplemented by the fact that variability gradually be- 

 came reduced during the progress of the inbreeding. 



The investigations of the effects of inbreeding on the 

 guinea-pig, to which we have referred, were begun in 

 1906 by G. M. Rommel of the United States Department 

 of Agriculture. In recent years the work has been in 

 charge of Sewall Wright, who has made a very illumi- 

 nating analysis of the results obtained. 



This series of experiments was started with thirty- 

 three pairs of stock animals which had been more or less 

 inbred previously. Although maintained exclusively by 

 mating sister with brother, sixteen of these families were 

 in existence at the close of 1917 after some twenty gen- 

 erations of the closest inbreeding. 



Considered as a whole this inbred race shows distinct 

 evidence of having declined in every character connected 

 with vigor. The litters are smaller and are produced 

 more irregularly. The per cent, of mortality both in utero 

 and between birth and weaning has increased. The birth 

 weights are lower and the rate of growth slower than in 

 control stock. In spite of these facts, however, one is 

 forced to the conclusion that these results are not the 

 effect of inbreeding as a direct cause, but are to be attrib- 

 uted to Mendelian segregation. 



There are pronounced differences between the various 

 families. Some are still very vigorous, comparing favor- 



