UNIT-CHARACTERS OF HORSES 135 



In horses it is a hay factor which the black variety has lost. 

 This factor appears to inhibit the development of black in 

 regions where the bay variety shows red, just as an agouti 

 factor inhibits the development of black pigment in certain 

 regions of the coat of rodents which then are yellow. \Mien 

 the bay factor is lacking, black pigment develops throughout 

 the entire coat. WTiether this loss occurred originally as a 

 single sudden change (a sport) or whether it occurred gradually 

 is uncertain, but it seems clear that at present in crosses black 

 is a unit-character recessive to bay, and this makes it seem 

 probable that it arose as a discontinuous variation originally. 



A unit-character difference has also been shown to exist 

 between black and chestnut horses, a difference comparable 

 to that which exists between black and brown varieties of 

 rodents. Chestnut is recessive to black, corresponding with 

 the " chocolate " varieties of rodents. " Suffolk " or " Suf- 

 folk Punch " horses are invariably chestnut in color. But 

 the term " chestnut " as here used probably includes both 

 brown annuals which, like black, lack the bay factor and those 

 which possess this factor. For the latter it would probably 

 be better to use a term in common use, sorrel. We should 

 then have parallel black and brown series with and without 

 the bay factor. Black pigmented horses with the bay factor 

 are " bays," without it they are " blacks." Brown pig- 

 mented horses with the bay factor should be called " sor- 

 rel "; those without it, chestnut. Records compiled by 

 Wentworth and others indicate that such a factorial differ- 

 ence does exist among horses called " chestnut " in the 

 records. For blacks mated inter se produce some chestnut 

 colts (which should be possible if the black parents are 

 heterozygous for chestnut) with a doubtful record of a few 

 bays, but black mated with " chestnut " produces more bays 

 than anything else, which shows clearly that some at least 

 of the chestnut parents do transmit the bay factor. 



The gray (or white) color variation of horses corresponds 

 roughly with the white variation in cattle. It is a dominant 

 unit-character in crosses, but shows itself only in the second 



