4 
SEXUAL SELECTIOX. 
Part II. 
the intermaxillary bones of the upper jaw.”^ (Figs. 
26 and 27.) In our salmon this change of structure 
lasts only during the breeding-season ; but in the Salmo 
hjcaodon of N.W. America the change, as Mr. J. 
Fig. 26. Head of male of commoa salmon {Salmo salar) during the breeding-season, 
[This drawing, as well as all the others in the present chapter, have been executed by 
the well-known artist, Mr. G. Ford, under the kind superintendence of Dr. Gunther, froni 
specimens in the British Museum.] 
^ Yarrell, ‘ History of British Fishes,’ vol. ii. 1836, p. 10. 
