Wilson — Pohjgamous lieiidelian Factors. 305 



Aud, iu a general population, containing pure and impure browns and blacks, 

 tlie matings of brown and black should produce all the four colours of the set, 

 This, however, browns and blacks do not do. They produce no fourth colour, 

 but only tliemselves and chestnut. What is the explanation ? Clearly there 

 are not two pairs of factors. Clearly also, since there are three colours, there 

 must be at least tliree factors. But how many factors are necessary for the 

 production of each colour and how do they combine with each other ? 



Consider first liow brown and black are related. In the Shire Stud-Book 

 a number of sires have been found which produce bay foals only whether 

 mated with brown or black mares; aud Dr. Walther found a number of 

 similar German sires. Thus brown is dominant to black. At Uie same time 

 a number of Shire stallions were found which produced both brown and 

 black foals when mated witli brown, and equal numbers of browns and blacks 

 when mated with black. Dr. Walther also found a number of German sires 

 of the same kind. Thus brown differs from black in one pair of characters. 

 The following table gives tlie actual figures. In both cases there are a few 

 exceptions, but they are so few that they may be put down as errors either of 

 description or parentage, and neglected. Horse colours are very liable to such 

 errors for the reason that most of the animals are entered in their stud-books 

 while they are foals, at which age dark brown is frequently mistaken for 

 black and bay for chestnut. In the table the exceptions are enclosed within 

 brackets. The column labelled "doubtful" includes such descriptions as 

 " brown or black " and " bay or chestnut " : — 



By these figures brown is dominant to black and, by the fact that equal 

 numbers of browns and blacks are produced when impure browns are mated 

 witli blacks, differs from it iu one pair of characters. 



Thus we have a series of three characters, chestnut, black, and brown, 

 eacli of which differs from the other two in one pair of characters and in 

 which chestnut is recessive to black and brown, and black recessive to brown. 



From this it follows that each of these three colours is the result of 

 one factor only ; for, if not, and each were the result of two factors ; then 



the constitutions would be, say, chestnut ^ '^ , black , and brown, yy' 



