126 THE COURTSHIP OF ANIMALS 



momentum gained, from some obscure cause, keeps on 

 being increased by cumulative inheritance : and not being 

 checked by Natural Selection, causes the species in 

 respect of such characters to pass beyond its congeners. 

 Professor Lloyd Morgan's theory of " over-production " 

 would seem better to apply here, though in a somewhat 

 different sense from that used by him. For in the 

 instances just q-uoted there is a latent potentiality for 

 response to new demands which the struggle for existence 

 may make, but a potentiality varying in degree, and here 

 selection finds its metier. 



Yet further illustrations of secondary sexual characters, 

 such as are concerned with vocal music, must now be 

 considered. The discussion of these has been designedly 

 deferred. They embrace instances of voice production 

 more singular than any yet referred to, and if possible 

 more difficult to interpret. 



The facts first to be reviewed concern the syrinx of 

 certain of the Anatidae. It is noteworthy that each 

 of the three divisions of this group — the Swans, Geese and 

 Ducks — contains species in which either the syrinx or the 

 windpipe has acquired some singular feature. In the 

 surface-feeding Ducks, modifications of the syrinx are 

 most frequently found. Commonly, as in the Mallard, 

 this takes the form of a spherical bony case ; in the diving 

 Ducks this bony chamber has enormously increased in 

 size. Furthermore it has conspicuously changed both 

 in form and character : for it is now roughly trihedral 

 in form, and its walls present large jenestrce closed only 

 by delicate membrane, suggesting that the increased 

 size of the chamber has not been accompanied by a 

 corresponding increase of bony tissue for its construction. 

 Hence all that is available is used for the construction 



