PRINCIPLES OF LIVESTOCK BREEDING. 41 



decline, and some decline is shown by the average for all families. 

 The great differences between families confirm the suggestion that 

 inbreeding is merely likely to lead to decline in vigor, but does not 

 necessarily do so. This conclusion is brought out also on consider- 

 ing the different characteristics se parately. One family lost markedly 

 in vitality but not in size or fertility. In another the reverse was 

 the case. In fact, nearly all combinations of favorable or unfavorable 

 characteristics were represented by one or more f amilies after a num- 

 ber of generations of inbreeding. 



The results of crosses between different inbred families are inter- 

 esting in this connection. The young from such crosses made dis- 

 tinctly better gains and a larger percentage were raised of those born 

 alive than in their inbred cousins raised at the same time under the 

 same conditions. The crossbred females have much larger litters 

 and have them more frequently. More of their young are born alive 

 and the birth weights are greater than of young born of inbred dams. 

 The second generation, in fact, appears to be as vigorous as a control 

 stock which has never been inbred . 



These results are easily interpreted by the present theory of hered- 

 ity. They confirm the view that the primary effect of inbreeding is 

 merely the automatic fixation of hereditary factors. It seems to be 

 the usual rule that factors favorable to vigor are dominant over 

 unfavorable ones, and hence tend to conceal the latter under cross- 

 breeding. Under inbreeding the unfavorable factors are as likely to 

 become fixed as favorable ones, and hence are brought to light. One 

 or more of the unfavorable factors affecting size, vitality, or fertility 

 are thus very likely to become fixed in each line, especially as it is 

 very likely to happen that some of the favorable and unfavorable 

 factors may be linked in their heredity, which means that the at- 

 tempt to fix these favorable factors involves an involuntary fixation 

 of the unfavorable ones. It usually happens that different defects 

 become fixed in different lines, so that on crossing each supplies the 

 elements of vigor which the other lacks and full vigor returns. 



The fixation of unfavorable characteristics can be prevented to 

 some extent by sufficiently careful selection, but it must be remem- 

 bered that fertility, vigor, etc., depend on so many factors besides 

 heredity that even the most careful selection will often be at fault. 

 Hereditary differences in these respects can not, in fact, be deter- 

 mined with certainty except by starting a large number of inbred 

 lines and comparing them. It is the discovery of one really valuable 

 line, out of a score or more of closely bred lines, which may be expected 

 to make history in livestock breeding. 



While the conclusions in regard to inbreeding as given above are 

 based on experiments with higher animals, it may be well to add 

 that extensive inbreeding of insects has given similar results and that 



