PRINCIPLES OF LIVESTOCK BREEDING. Si 



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 



