COURTSHIP OF THE REDSHANK. 655 



(1) The markings used in display (red legs, white tail, and 

 whitish under surface of wings) Avere acquired previously by the 

 species for some distinct purpose {e. g. as recognition or oblitera- 

 tion markings) or possibly accidentally, and then were tuxned to 

 account by the cock as the "physical basis " of his display. The 

 hen would of course possess the markings too ; she difi'ers from 

 the male in not having secondarily acquired the instinct to display 

 them. 



(2) The markings were acquired by the cock first or primarily 

 as secondary sexual characters, to form the basis of his display, 

 but were either at once or later found to be of use in other ways : 

 they would then tend to be transferred to the hen as well, either 

 from their first beginnings, or subsequently to their definitive 

 development in the cock, by the operation of Natiu-al Selection. 



(3) The markings were acquired for purposes of display, while 

 in other respects they are neither harmful nor the reverse ; the 

 instinct to use them for display, however, depends on a physio- 

 logical stimulus only present in the male sex. Then they would 

 tend to be transferred to the female sex, for we generally find 

 that the two sexes resemble each other unless there is some 

 definite reason for their diflering. It appears to be both more 

 primitive and easier for hereditary characters to be transmitted 

 equally to both sexes. 



It is at present very hard to decide between these possibilities. 

 Of the three, the last appears the least probable. What does 

 emerge clearly, however, is that in considering the facts of Sexual 

 Selection, as so often elsewhere, we must be careful not to isolate 

 structure from function. When we speak of secondary sexual 

 characters, we usually think of structures only. In reality the real 

 character is the structure plus the instinct to use the structure, 

 for it is the use of the structure which alone has any significance 

 for the species : it is that which constitutes a unity, it is that 

 which has been really acquired by the species. For purposes of 

 convenience we separate it into two components — structure and 

 function; but in any question of its origin and history we must 

 always be careful to think of it as a whole. To take a concrete 

 example : if it were proved (as is probable) that the female Red- 

 shank never used her white tail, etc. for purposes of display, we 

 should be justified in saying that the Redshank showed secondary 

 sexual characters — these characters being the various actions of 

 display found in the male, and in the male alone. 



This point of view will perhaps help to make more intelligible 

 the vaiious cases which have been described where the sexes are 

 alike in plumage, but the male alone goes through a display. 



