PRINCIPLES OF LIVESTOCK BREEDING. 41 
often pictured. Other families, however, suffered a rapid decline, 
and some decline is shown by the average for all families. The 
great differences between families confirm the suggestion that inbreed- 
ing is merely likely to lead to decline in vigor, but does not necessarily 
do so. This conclusion is brought out also on considering the different 
characteristics separately. 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 character- 
istics were represented by one or more families after a number 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 
