PRINCIPLES OF LIVESTOCK BREEDING. 37 
of hooded rats and Dutch rabbits are much less likely to be asym- 
metrical than that of piebald guinea pigs, and it is found that a given 
pattern can be fixed in them much more perfectly. The white face of 
Hereford cattle is usually symmetrical and has been fixed to a satis- 
factory extent. Whether the white belt of Hampshire swine can be 
so fixed seems more doubtful, owing to its frequent asymmetry. 
Occasionally a variation is due to the appearance of a wholly new 
hereditary characteristic in a stock. The polled variation of cattle 
has probably appeared in this way a number of times. Such varia- 
tions, or mutations, as they are called, are, however, very rare. 
Most hereditary variation is due simply to recombination of the 
factors already present in the parent stocks. The blue roans and 
their varied progeny, derived from crosses between Shorthorn and 
Aberdeen-Angus cattle, are a good illustration of variation of this 
sort. It is this form of variation only which can be eliminated by 
methods of breeding. 
FIXATION OF HEREDITY BY SELECTION. 
Consistent selection toward the desired type is sometimes all that 
is necessary to fix a characteristic. Unfortunately, experiments 
have shown that what appear to be the same characteristics in two 
animals often depend on wholly different combinations of hereditary 
factors. A good example has been given in another connection in 
the case of two strains of light-eyed, yellow rats, each of which bred 
true by itself, but which produced nothing but black-eyed gray rats 
when crossed with each other. Thus progress by straight selection 
may be wholly upset at any time by an unfortunate cross of this 
kind. The whole breed must be lifted up at once if there is to be 
success by selection alone. Careful selection with breeding confined 
within a single herd or a few related herds, on the other hand, only 
requires that this small group be lifted up at once. Once success 
has been obtained, such a herd or group of herds becomes a powerful 
source of breed improvement by supplying prepotent sires. Prac- 
tical experience agrees with theory in the principle that the only 
systematic method of fixing heredity, and so bringing out such 
prepotency as is in a stock, is Bakewell's old method of close breeding 
accompanied by careful selection. 
FIXATION OF HEREDITY BY INBREEDING. 
The primary effect of inbreeding is the fixation of hereditary 
qualities, whether good, bad, or indifferent. In other words, a 
sufficiently inbred animal produces only one kind of reproductive 
cell with respect to all hereditary characteristics (with the exception 
of sex and characters linked with sex in the case of male mammals 
