68 THE GENETIC AND THE OPERATIVE EVIDENCE 



we can account for the hornless daughters, and if other mothers did 

 not have this factor (or were heterozygous for it) we can account for 

 the horned daughters. Evidently more evidence is needed. Arkell 

 himself assigns a corresponding difference to the mothers in these cases, 

 based on the observed fact that the mother that had knobs or scurs 

 were the ones that gave birth to the horned daughters. If the above 

 suggestion proves true, it shows that the Merino condition dominates 

 the Dorset condition. The result is in harmony with the view that 

 both have a common factor for horns, but that in addition the Merinos 

 have a non-sex-linked modifier that holds down the development of the 

 horns in the ewe. 



What bearing have these results on the theory of sexual selection? 

 Clearly the Merino male, as constituted at present, develops horns 

 because he is a male, but only in the sense that his testes secrete some 

 substance that makes his horns grow. That maleness does not in itself 

 necessarily produce horn is shown by the absence of horns in the 

 Suffolk breed. Is it the same factor, present in the Merino, that 

 produces horns in both sexes of Dorsets when homozygous and in the 

 male only when heterozygous? If originally the ancestral race had no 

 horns, the appearance of factors for horns would, even in a heterozygous 

 condition, have sufficed in the males for the development of horns. 

 If this gave them any advantage either over the enemies of the race or 

 in the eyes of the female, such factors might be perpetuated, and 

 through transference to the females ultimately become homozygous 

 in both sexes. Both would then have horns, whether horns were or 

 were not of any advantage to the female, which would have them 

 because they have an advantage to the other sex. 



Because the genetic evidence shows that a single factor difference 

 between the breeds with and without horns accounts for the horned 

 condition in one of them, it by no means follows that horns as they 

 exist arose as a single mutant factor change. True, they may have 

 arisen as a new single factor difference, but the Mendelian evidence 

 can not be claimed as evidence for this view. The a priori argument 

 based on the relation of horns in an adaptive sense to the rest of the 

 body would appear rather to indicate that they could not have arisen 

 at a single mutational step. 



Concerning the still broader bearing of this evidence on the theory 

 of sexual selection, two distinct questions are involved : first, how has 

 the present racial difference in horns arisen in domesticated sheep, and 

 secondly, what was the original condition of sheep. Reversing the 

 order of these questions, we find that sheep were domesticated in Asia 

 and Europe before the dawn of history. "Whether our well-known 

 and useful animal is derived from any one of the existing wild species, 

 or from the crossing of several, or from some now extinct species, is 

 quite a matter of conjecture" (Flower and Lydekker's "Mammals"). 



