66 THE GENETIC AND THE OPERATIVE EVIDENCE 



the same breed is claimed by Arkell as a result of a large number of 

 crosses that he has carried out. He states, for instance, that in the 

 great Merino class, with its various sub-breeds, there are flocks in 

 which the males only are homed, but even here there may be individual 

 males that are hornless "and at times the females may also show some 

 signs of horn growth." In America, Arkell states, there are three 

 types of Merinos — the American, the Delaine, and the Rambouillet. 

 He quotes Plumb (Types and Breeds of Farm Animals, Boston, 1906) 

 as stating that "the American Merino ram carries heavy, spirally 

 twisted horns, but the ewes are hornless; .... that the rams of the 

 National Standard or Victor-Beald Delaines may or may not have 

 horns ; that the Dickinson Delaines may have small horns, but a polled 

 head is preferred," etc. These conditions suggest that there may be 

 more than a single factor for horns in sheep or that there may be 

 modifying factors in certain breeds. In fact, Arkell and Davenport 

 attempt to cover the results of Arkell's experiments by assuming that 

 there is an inhibiting factor for horns that is carried by the sex chromo- 

 some. Such an inhibitor (I) would be double in the XX female and 

 single in the X male. It is assumed to be incapable of preventing the 

 development of horns in the heterozygous Hh male, the inhibitor being 

 there simplex (i. e., one I), while the double inhibitoris capable of prevent- 

 ing the horns in the heterozygous (Hh) condition, but not of preventing 

 the development of horns when the homozygous (HH) condition 

 occurs. There are several objections to this scheme: first, that there is 

 no evidence that a sex-linked inhibitor is present that affects the horn- 

 less breeds, for the evidence indicates rather that there is no factor for 

 horns present in them, at least in the Suffolks; second, the peculiar 

 balance between the factors for horns and the inhibitor seems an 

 extremely artificial statement. Arkell and Davenport intimate that 

 races with horned males and hornless fenaales do not exist in a pure 

 state. That breeds impure in these respects may exist need not be 

 denied, but that pure races for such a dimorphic condition do exist 

 seems probable. Castle states, for instance, that he knows at first 

 hand of such races of Merinos. Castle also states that castrated Merino 

 rams in this race do not develop horns, and this result is in accordance 

 with statements made by Marshall for Herdwicks (a race with horned 

 males and hornless females). Under the circumstances it is certain 

 that the presence of the testes is one of the factors in determining 

 whether horns develop at all (as in Merinos), or in determining the 

 extent to which they develop (as in the Dorsets), rather than that the 

 difference between the sexes is due only to an inhibiting genetic factor. 

 Nevertheless, it may be well to keep open the possibiUty that there may 

 be different factors for horns in different races (allelomorphs or others), 

 or conversely, that the genetic composition of the races is different, the 

 factor for horns remaining the same, but producing a different effect. 



