128 ORIGIN OF SOMATIC 



complicated theory of sexual selection. The possi- 

 bility of a Lamarckian explanation he does not even 

 mention. He would doubtless assume that the 

 antlers of stagig arose as a mutation, without ex- 

 plaining how they came to be affected by the testi- 

 cular hormone, and that when they arose the stags 

 found them convenient as fighting weapons. But 

 the complicated adaptive relations are not to be 

 disposed of by the simple word mutation. The 

 males have sexual instincts, themselves dependent 

 on the testicular hormone, which develop sexual 

 jealousy and rivalr}^, and the Ruminants fight by 

 butting with their heads because they have no 

 incisor teeth in the upper jaw, or tusks, which 

 are used in fighting in other species. Doubtless, 

 mutations have occurred in antlers as in other char- 

 acters ; in fact all hereditary characters are subject 

 to mutation. This is the most probable explanation, 

 not only of the occasional occurrence of hornless 

 individual stags, but of the differences between the 

 antlers of different species, for there is no reason to 

 believe that the special character of the antler in 

 each species is adapted to a special mode of fighting 

 in each species. 



The different structure of the horns of the Bovine 

 and Ovine Ruminants is, in my view, the result of a 

 different mode of fighting. If we suppose that the 

 fighting was slower and less fierce in the Bovidae, 

 so that the skin over the exostosis was subject to 

 friction but not lacerated, the result would be a 

 thickening of the horny layer of the epidermis as we 

 find it, and the fact that the skin and periosteum are 

 not destroyed explains why the horns are not shed 

 but permanent. 



i 



